Exterior Paint Problems on Historic
Woodwork
Kay D. Weeks and David W. Look, AIA
A cautionary approach to paint removal is included in the guidelines
to the Secretary of the Interior Standards for Rehabilitation.
Removing paints down to bare wood surfaces using harsh methods can permanently
damage those surfaces; therefore such methods are not recommended. Also,
total removal obliterates evidence of the historical paints and their
sequence and architectural context.
This Brief expands on that advice for the architect, building manager,
contractor, or homeowner by identifying and describing common types
of paint surface conditions and failures, then recommending appropriate
treatments for preparing exterior wood surfaces for repainting (1) to
assure the best adhesion and greatest durability of the new paint. Although
the Brief focuses on responsible methods of "paint removal," several
paint surface conditions will be described which do not require any
paint removal, and still others which can be successfully handled by
limited paint removal. In all cases, the information is intended to
address the concerns related to exterior wood. It will also be generally
assumed that, because houses built before 1950 involve one or more layers
of leadbase paint,(2) the majority of conditions warranting paint removal
will mean dealing with this toxic substance along with the dangers of
the paint removal tools and chemical strippers themselves.
Purposes of Exterior Paint
Paint (3) applied to exterior wood must withstand yearly extremes
of both temperature and humidity. While never expected to be more than
a temporary physical shield--requiring reapplication every 58 years--its
importance should not be minimized. Because one of the main causes of
wood deterioration is moisture penetration, a primary purpose for painting
wood is to exclude such moisture, thereby slowing deterioration not
only of a building's exterior siding and decorative features but, ultimately,
its underlying structural members. Another important purpose for painting
wood is, of course, to define and accent architectural features and
to improve appearance.
Treating Paint Problems in Historic
Buildings
Exterior paint is constantly deteriorating through the processes of
weathering, but in a program of regular maintenance--assuming all other
building systems are functioning properly--surfaces can be cleaned,
lightly scraped, and hand sanded in preparation for a new finish coat.
Unfortunately, these are ideal conditions. More often, complex maintenance
problems are inherited by owners of historic buildings, including areas
of paint that have failed (4) beyond the point of mere cleaning, scraping,
and hand sanding (although much so-called "paint failure" is attributable
to interior or exterior moisture problems or surface preparation and
application mistakes with previous coats).
Although paint problems are by no means unique to historic buildings,
treating multiple layers of hardened, brittle paint on complex, ornamental--and
possibly fragile--exterior wood surfaces necessarily requires an extremely
cautious approach (see figure 1). In the case of recent construction,
this level of concern is not needed because the wood is generally less
detailed and, in addition, retention of the sequence of paint layers
as a partial record of the building's history is not an issue.
When historic buildings are involved, however, a special set of problems
arises--varying in complexity depending upon their age, architectural
style, historical importance, and physical soundness of the wood--which
must be carefully evaluated so that decisions can be made that are sensitive
to the longevity of the resource.
Justification for Paint Removal
At the outset of this Brief, it must be emphasized that removing paint
from historic buildings--with the exception of cleaning, light scraping,
and hand sanding as part of routine maintenance--should be avoided unless
absolutely essential. Once conditions warranting removal have been identified
the general approach should be to remove paint to the next sound layer
using the gentlest means possible, then to repaint (see figure 2). Practically
speaking as well, paint can adhere just as effectively to existing paint
as to bare wood, providing the previous coats of paint are also adhering
uniformly and tightly to the wood and the surface is properly prepared
for repainting-- cleaned of dirt and chalk and dulled by sanding. But,
if painted exterior wood surfaces display continuous patterns of deep
cracks or if they are extensively blistering and peeling so that bare
wood is visible, then the old paint should be completely removed before
repainting. The only other justification for removing all previous layers
of paint is if doors, shutters, or windows have literally been "painted
shut," or if new wood is being pieced-in adjacent to old painted wood
and a smooth transition is desired (see figure 3).
Paint Removal Precautions
Because paint removal is a difficult and painstaking process, a number
of costly, regrettable experiences have occurred--and continue to occur--for
both the historic building and the building owner. Historic buildings
have been set on fire with blow torches; wood irreversibly scarred by
sandblasting or by harsh mechanical devices such as rotary sanders and
rotary wire strippers; and layers of historic paint inadvertently and
unnecessarily removed. In addition, property owners, using techniques
that substitute speed for safety, have been injured by toxic lead vapors
or dust from the paint they were trying to remove or by misuse of the
paint removers themselves.
Owners of historic properties considering paint removal should also
be aware of the amount of time and labor involved. While removing damaged
layers of paint from a door or porch railing might be readily accomplished
within a reasonable period of time by one or two people, removing paint
from larger areas of a building can, without professional assistance,
easily become unmanageable and produce less than satisfactory results.
The amount of work involved in any paint removal project must therefore
be analyzed on a case-by-case basis. Hiring qualified professionals
will often be a cost-effective decision due to the expense of materials,
the special equipment required, and the amount of time involved. Further,
paint removal companies experienced in dealing with the inherent health
and safety dangers of paint removal should have purchased such protective
devices as are needed to mitigate any dangers and should also be aware
of State or local environmental and/or health regulations for hazardous
waste disposal.
All in all, paint removal is a messy, expensive, and potentially dangerous
aspect of rehabilitating or restoring historic buildings and should
not be undertaken without careful thought concerning first, its necessity,
and second, which of the available recommended methods is the safest
and most appropriate for the job at hand.
Repainting
Historic Buildings for Cosmetic Reasons
If existing exterior paint on wood siding, eaves, window sills, sash,
and shutters, doors, and decorative features shows no evidence of paint
deterioration such as chalking, blistering, peeling, or cracking, then
there is no physical reason to repaint, much less remove paint! Nor
is color fading, of itself, sufficient justification to repaint a historic
building.
The decision to repaint may not be based altogether on paint failure.
Where there is a new owner, or even where ownership has remained constant
through the years, taste in colors often changes. Therefore, if repainting
is primarily to alter a building's primary and accent colors, a technical
factor of paint accumulation should be taken into consideration. When
paint builds up to a thickness of approximately 1/16~ (approximately
1630 layers), one or more extra coats of paint may be enough to trigger
cracking and peeling in limited or even widespread areas of the building's
surface. This results because excessively thick paint is less able to
withstand the shrinkage or pull of an additional coat as it dries and
is also less able to tolerate thermal stresses. Thick paint invariably
fails at the weakest point of adhesion--the oldest layers next to the
wood. Cracking and peeling follow. Therefore, if there are no signs
of paint failure, it may be somewhat risky to add still another layer
of unneeded paint simply for color's sake (extreme changes in color
may also require more than one coat to provide proper hiding power and
full color). When paint appears to be nearing the critical thickness,
a change of accent colors (that is, just to limited portions of the
trim) might be an acceptable compromise without chancing cracking and
peeling of paint on wooden siding.
If the decision to repaint is nonetheless made, the "new" color or
colors should, at a minimum, be appropriate to the style and setting
of the building. On the other hand, where the intent is to restore or
accurately reproduce the colors originally used or those from a significant
period in the building's evolution, they should be based on the results
of a paint analysis.(5)
Identification of Exterior Paint Surface
Conditions/Recommended Treatments
It is assumed that a preliminary check will already have been made
to determine, first, that the painted exterior surfaces are indeed wood--and
not stucco, metal, or other wood substitutes--and second, that the wood
has not decayed so that repainting would be superfluous. For example,
if any area of bare wood such as window sills has been exposed for a
long period of time to standing water, wood rot is a strong possibility
(see figure 4). Repair or replacement of deteriorated wood should take
place before repainting. After these two basic issues have been resolved,
the surface condition identification process may commence.
The historic building will undoubtedly exhibit a variety of exterior
paint surface conditions. For example, paint on the wooden siding and
doors may be adhering firmly; paint on the eaves peeling; and paint
on the porch balusters and window sills cracking and alligatoring. The
accurate identification of each paint problem is therefore the first
step in planning an appropriate overall solution.
Paint surface conditions can be grouped according to their relative
severity: CLASS I conditions include minor blemishes or dirt collection
and generally require no paint removal; CLASS II conditions include
failure of the top layer or layers of paint and generally require limited
paint removal; and CLASS III conditions include substantial or multiple-layer
failure and generally require total paint removal. It is precisely because
conditions will vary at different points on the building that a careful
inspection is critical. Each item of painted exterior woodwork (i.e.,
siding, doors, windows, eaves, shutters, and decorative elements) should
be examined early in the planning phase and surface conditions noted.
CLASS I Exterior Surface Conditions Generally Requiring No Paint
Removal
* Dirt, Soot, Pollution, Cobwebs, Insect Cocoons, etc.
Cause of Condition
Environmental "grime" or organic matter that tends to cling to painted
exterior surfaces and, in particular, protected surfaces such as eaves,
do not constitute a paint problem unless painted over rather than removed
prior to repainting. If not removed, the surface deposits can be a barrier
to proper adhesion and cause peeling.
Recommended Treatment
Most surface matter can be loosened by a strong, direct stream of
water from the nozzle of a garden hose. Stubborn dirt and soot will
need to be scrubbed off using l/2 cup of household detergent in a gallon
of water with a medium soft bristle brush. The cleaned surface should
then be rinsed thoroughly, and permitted to dry before further inspection
to determine if repainting is necessary. Quite often, cleaning provides
a satisfactory enough result to postpone repainting.
* Mildew
Cause of Condition
Mildew is caused by fungi feeding on nutrients contained in the paint
film or on dirt adhering to any surface. Because moisture is the single
most important factor in its growth, mildew tends to thrive in areas
where dampness and lack of sunshine are problems such as window sills,
under eaves, around gutters and downspouts, on the north side of buildings,
or in shaded areas near shrubbery. It may sometimes be difficult to
distinguish mildew from dirt, but there is a simple test to differentiate:
if a drop of household bleach is placed on the suspected surface, mildew
will immediately turn white whereas dirt will continue to look like
dirt.
Recommended Treatment
Because mildew can only exist in shady, warm, moist areas, attention
should be given to altering the environment that is conducive to fungal
growth. The area in question may be shaded by trees which need to be
pruned back to allow sunlight to strike the building; or may lack rain
gutters or proper drainage at the base of the building. If the shady
or moist conditions can be altered, the mildew is less likely to reappear.
A recommend solution for removing mildew consists of one cup non-ammoniated
detergent, one quart household bleach, and one gallon water. When the
surface is scrubbed with this solution using a medium soft brush, the
mildew should disappear; however, for particularly stubborn spots, an
additional quart of bleach may be added. After the area is mildew-free,
it should then be rinsed with a direct stream of water from the nozzle
of a garden hose, and permitted to dry thoroughly. When repainting,
specially formulated "mildew-resistant" primer and finish coats should
be used.
* Excessive Chalking
Cause of Condition
Chalking--or powdering of the paint surface--is caused by the gradual
disintegration of the resin in the paint film. (The amount of chalking
is determined both by the formulation of the paint and the amount of
ultraviolet light to which the paint is exposed.) In moderation, chalking
is the ideal way for a paint to "age," because the chalk, when rinsed
by rainwater, carries discoloration and dirt away with it and thus provides
an ideal surface for repainting. In excess, however, it is not desirable
because the chalk can wash down onto a surface of a different color
beneath the painted area and cause streaking as well as rapid disintegration
of the paint film itself. Also, if a paint contains too much pigment
for the amount of binder (as the old white lead carbonate/oil paints
often did), excessive chalking can result.
Recommended Treatment
The chalk should be cleaned off with a solution of l/2 cup household
detergent to one gallon water, using a medium soft bristle brush. After
scrubbing to remove the chalk, the surface should be rinsed with a direct
stream of water from the nozzle of a garden hose, allowed to dry thoroughly,
(but not long enough for the chalking process to recur) and repainted,
using a non-chalking paint.
* Staining
Cause of Condition
Staining of paint coatings usually results from excess moisture reacting
with materials within the wood substrate. There are two common types
of staining, neither of which requires paint removal. The most prevalent
type of stain is due to the oxidation or rusting of iron nails or metal
(iron, steel, or copper) anchorage devices. A second type of stain is
caused by a chemical reaction between moisture and natural extractives
in certain woods (red cedar or redwood) which results in a surface deposit
of colored matter. This is most apt to occur in new replacement wood
within the first 1015 years.
Recommended Treatment
In both cases, the source of the stain should first be located and
the moisture problem corrected.
When stains are caused by rusting of the heads of nails used to attach
shingles or siding to an exterior wall or by rusting or oxidizing iron,
steel, or copper anchorage devices adjacent to a painted surface, the
metal objects themselves should be hand sanded and coated with a rust-inhibitive
primer followed by two finish coats. (Exposed nail heads should ideally
be countersunk, spot primed, and the holes filled with a high quality
wood filler except where exposure of the nail head was part of the original
construction system or the wood is too fragile to withstand the countersinking
procedure.)
Discoloration due to color extractives in replacement wood can usually
be cleaned with a solution of equal parts denatured alcohol and water.
After the affected area has been rinsed and permitted to dry, a "stainblocking
primer" especially developed for preventing this type of stain should
be applied (two primer coats are recommended for severe cases of bleeding
prior to the finish coat). Each primer coat should be allowed to dry
at least 48 hours.
CLASS II Exterior Surface Conditions Generally Requiring Limited
Paint Removal
* Crazing
Cause of Condition
Crazing--fine, jagged interconnected breaks in the top layer of paint--results
when paint that is several layers thick becomes excessively hard and
brittle with age and is consequently no longer able to expand and contract
with the wood in response to changes in temperature and humidity (see
figure 5). As the wood swells, the bond between paint layers is broken
and hairline cracks appear. Although somewhat more difficult to detect
as opposed to other more obvious paint problems, it is well worth the
time to scrutinize all surfaces for crazing. If not corrected, exterior
moisture will enter the crazed surface, resulting in further swelling
of the wood and, eventually, deep cracking and alligatoring, a Class
III condition which requires total paint removal.
Recommended Treatment
Crazing can be treated by hand or mechanically sanding the surface,
then repainting. Although the hairline cracks may tend to show through
the new paint, the surface will be protected against exterior moisture
penetration.
* Intercoat Peeling
Cause of Condition
Intercoat peeling can be the result of improper surface preparation
prior to the last repainting. This most often occurs in protected areas
such as eaves and covered porches because these surfaces do not receive
a regular rinsing from rainfall, and salts from airborne pollutants
thus accumulate on the surface. If not cleaned off, the new paint coat
will not adhere properly and that layer will peel.
Another common cause of intercoat peeling is incompatibility between
paint types (see figure 6). For example, if oil paint is applied over
latex paint, peeling of the top coat can sometimes result since, upon
aging, the oil paint becomes harder and less elastic than the latex
paint. If latex paint is applied over old, chalking oil paint, peeling
can also occur because the latex paint is unable to penetrate the chalky
surface and adhere.
Recommended Treatment
First, where salts or impurities have caused the peeling, the affected
area should be washed down thoroughly after scraping, then wiped dry.
Finally, the surface should be hand or mechanically sanded, then repainted.
Where peeling was the result of using incompatible paints, the peeling
top coat should be scraped and hand or mechanically sanded. Application
of a high quality oil type exterior primer will provide a surface over
which either an oil or a latex topcoat can be successfully used.
* Solvent Blistering
Cause of Condition
Solvent blistering, the result of a less common application error,
is not caused by moisture, but by the action of ambient heat on paint
solvent or thinners in the paint film. If solventrich paint is applied
in direct sunlight, the top surface can dry too quickly and, as a result,
solvents become trapped beneath the dried paint film. When the solvent
vaporizes, it forces its way through the paint film, resulting in surface
blisters. This problem occurs more often with dark colored paints because
darker colors absorb more heat than lighter ones. To distinguish between
solvent blistering and blistering caused by moisture, a blister should
be cut open. If another layer of paint is visible, then solvent blistering
is likely the problem whereas if bare wood is revealed, moisture is
probably to blame. Solvent blisters are generally small.
Recommended Treatment
Solvent-blistered areas can be scraped, hand or mechanically sanded
to the next sound layer, then repainted. In order to prevent blistering
of painted surfaces, paint should not be applied in direct sunlight.
* Wrinkling
Cause of Condition
Another error in application that can easily be avoided is wrinkling
(see figure 7). This occurs when the top layer of paint dries before
the layer underneath. The top layer of paint actually moves as the paint
underneath (a primer, for example) is drying. Specific causes of wrinkling
include: (1) applying paint too thick; (2) applying a second coat before
the first one dries; (3) inadequate brushing out; and (4) painting in
temperatures higher than recommended by the manufacturer.
Recommended Treatment
The wrinkled layer can be removed by scraping followed by hand or
mechanical sanding to provide as even a surface as possible, then repainted
following manufacturer's application instructions.
CLASS III Exterior Surface Conditions Generally Requiring Total Paint
Removal
If surface conditions are such that the majority of paint will have
to be removed prior to repainting, it is suggested that a small sample
of intact paint be left in an inconspicuous area either by covering
the area with a metal plate, or by marking the area and identifying
it in some way. (When repainting does take place, the sample should
not be painted over). This will enable future investigators to have
a record of the building's paint history.
* Peeling
Cause of Condition
Peeling to bare wood is most often caused by excess interior or exterior
moisture that collects behind the paint film, thus impairing adhesion
(see figure 8). Generally beginning as blisters, cracking and peeling
occur as moisture causes the wood to swell, breaking the adhesion of
the bottom layer.
Recommended Treatment
There is no sense in repainting before dealing with the moisture problems
because new paint will simply fail. Therefore, the first step in treating
peeling is to locate and remove the source or sources of the moisture,
not only because moisture will jeopardize the protective coating of
paint but because, if left unattended, it can ultimately cause permanent
damage to the wood. Excess interior moisture should be removed from
the building through installation of exhaust fans and vents. Exterior
moisture should be eliminated by correcting the following conditions
prior to repainting: faulty flashing; leaking gutters; defective roof
shingles; cracks and holes in siding and trim; deteriorated caulking
in joints and seams; and shrubbery growing too close to painted wood.
After the moisture problems have been solved, the wood must be permitted
to dry out thoroughly. The damaged paint can then be scraped off with
a putty knife, hand or mechanically sanded, primed, and repainted.
* Cracking/ Alligatoring
Cause of Condition
Cracking and alligatoring are advanced stages of crazing (see figure
9). Once the bond between layers has been broken due to intercoat paint
failure, exterior moisture is able to penetrate the surface cracks,
causing the wood to swell and deeper cracking to take place.
This process continues until cracking, which forms parallel to grain,
extends to bare wood. Ultimately, the cracking becomes an overall pattern
of horizontal and vertical breaks in the paint layers that looks like
reptile skin; hence, "alligatoring." In advanced stages of cracking
and alligatoring, the surfaces will also flake badly.
Recommended Treatment
If cracking and alligatoring are present only in the top layers they
can probably be scraped, hand or mechanically sanded to the next sound
layer, then repainted. However, if cracking and/or alligatoring have
progressed to bare wood and the paint has begun to flake, it will need
to be totally removed. Methods include scraping or paint removal with
the electric heat plate, electric heat gun, or chemical strippers, depending
on the particular area involved. Bare wood should be primed within 48
hours then repainted.
Selecting
the Appropriate/Safest Method to Remove Paint
After having presented the "hierarchy" of exterior paint surface conditions--from
a mild condition such as mildewing which simply requires cleaning prior
to repainting to serious conditions such as peeling and alligatoring
which require total paint removal--one important thought bears repeating:
if a paint problem has been identified that warrants either limited
or total paint removal, the gentlest method possible for the particular
wooden element of the historic building should be selected from the
many available methods.
The treatments recommended--based upon field testing as well as onsite
monitoring of Department of Interior grant-in-aid and certification
of rehabilitation projects--are therefore those which take three overriding
issues into consideration (1) the continued protection and preservation
of the historic exterior woodwork; (2) the retention of the sequence
of historic paint layers; and (3) the health and safety of those individuals
performing the paint removal. By applying these criteria, it will be
seen that no paint removal method is without its drawbacks and all recommendations
are qualified in varying degrees.
Methods for Removing Paint
After a particular exterior paint surface condition has been identified,
the next step in planning for repainting--if paint removal is required--is
selecting an appropriate method for such removal.
The method or methods selected should be suitable for the specific
paint problem as well as the particular wooden element of the building.
Methods for paint removal can be divided into three categories (frequently,
however, a combination of the three methods is used). Each method is
defined below, then discussed further and specific recommendations made:
Abrasive--"Abrading" the painted surface by manual and/or mechanical
means such as scraping and sanding. Generally used for surface preparation
and limited paint removal.
Thermal--Softening and raising the paint layers by applying heat followed
by scraping and sanding. Generally used for total paint removal.
Chemical--Softening of the paint layers with chemical strippers followed
by scraping and sanding. Generally used for total paint removal.
* Abrasive Methods (Manual)
If conditions have been identified that require limited paint removal
such as crazing, intercoat peeling, solvent blistering, and wrinkling,
scraping and hand sanding should be the first methods employed before
using mechanical means. Even in the case of more serious conditions
such as peeling--where the damaged paint is weak and already sufficiently
loosened from the wood surface --scraping and hand sanding may be all
that is needed prior to repainting.
Recommended Abrasive Methods (Manual)
Putty Knife/Paint Scraper: Scraping is usually accomplished with either
a putty knife or a paint scraper, or both. Putty knives range in width
from one to six inches and have a beveled edge. A putty knife is used
in a pushing motion going under the paint and working from an area of
loose paint toward the edge where the paint is still firmly adhered
and, in effect,"beveling" the remaining layers so that as smooth a transition
as possible is made between damaged and undamaged areas (see figure
10).
Paint scrapers are commonly available in 1-5/16, 2-1/2, and 3-1/2
inch widths and have replaceable blades. In addition, profiled scrapers
can be made specifically for use on moldings. As opposed to the putty
knife, the paint scraper is used in a pulling motion and works by raking
the damaged areas of paint away.
The obvious goal in using the putty knife or the paint scraper is
to selectively remove the affected layer or layers of paint; however,
both of these tools, particularly the paint scraper with its hooked
edge, must be used with care to properly prepare the surface and to
avoid gouging the wood.
Sandpaper/Sanding Block/Sanding sponge: After manually removing the
damaged layer or layers by scraping, the uneven surface (due to the
almost inevitable removal of varying numbers of paint layers in a given
area) will need to be smoothed or "feathered out" prior to repainting.
As stated before, hand sanding, as opposed to harsher mechanical sanding,
is recommended if the area is relatively limited. A coarse grit, open-coat
flint sandpaper--the least expensive kind--is useful for this purpose
because, as the sandpaper clogs with paint it must be discarded and
this process repeated until all layers adhere uniformly.
Blocks made of wood or hard rubber and covered with sandpaper are
useful for handsanding flat surfaces. Sanding sponges--rectangular sponges
with an abrasive aggregate on their surfaces--are also available for
detail work that requires reaching into grooves because the sponge easily
conforms to curves and irregular surfaces. All sanding should be done
with the grain.
Summary of Abrasive Methods (Manual)
Recommended: Putty knife, paint scraper, sandpaper, sanding block,
sanding sponge.
Applicable areas of building: All areas. For use on: Class I, Class
II, and Class III conditions.
Health/Safety factors: Take precautions against lead dust, eye damage;
dispose of lead paint residue properly.
* Abrasive Methods (Mechanical)
If hand sanding for purposes of surface preparation has not been productive
or if the affected area is too large to consider hand sanding by itself,
mechanical abrasive methods, i.e., power-operated tools may need to
be employed; however, it should be noted that the majority of tools
available for paint removal can cause damage to fragile wood and must
be used with great care.
Recommended Abrasive Methods (Mechanical)
Orbital sander: Designed as a finishing or smoothing tool--not for
the removal of multiple layers of paint--the orbital sander is thus
recommended when limited paint removal is required prior to repainting.
Because it sands in a small diameter circular motion (some models can
also be switched to a back-and-forth vibrating action), this tool is
particularly effective for "feathering" areas where paint has first
been scraped (see figure 11). The abrasive surface varies from about
3x7 inches to 4x9 inches and sandpaper is attached either by clamps
or sliding clips. A medium grit, open-coat aluminum oxide sandpaper
should be used; fine sandpaper clogs up so quickly that it is ineffective
for smoothing paint.
Belt sander: A second type of power tool--the belt sander--can also
be used for removing limited layers of paint but, in this case, the
abrasive surface is a continuous belt of sandpaper that travels at high
speeds and consequently offers much less control than the orbital sander.
Because of the potential for more damage to the paint or the wood, use
of the belt sander (also with a medium grit sandpaper) should be limited
to flat surfaces and only skilled operators should be permitted to operate
it within a historic preservation project.
Not Recommended
Rotary Drill Attachments: Rotary drill attachments such as the rotary
sanding disc and the rotary wire stripper should be avoided. The disc
sander--usually a disc of sandpaper about 5 inches in diameter secured
to a rubber based attachment which is in turn connected to an electric
drill or other motorized housing--can easily leave visible circular
depressions in the wood which are difficult to hide, even with repainting.
The rotary wire stripper--clusters of metals wires similarly attached
to an electric drill-type unit--can actually shred a wooden surface
and is thus to be used exclusively for removing corrosion and paint
from metals.
Waterblasting: Waterblasting above 600 p.s.i. to remove paint is not
recommended because it can force water into the woodwork rather than
cleaning loose paint and grime from the surface; at worst, high pressure
waterblasting causes the water to penetrate exterior sheathing and damages
interior finishes. A detergent solution, a medium soft bristle brush,
and a garden hose for purposes of rinsing, is the gentlest method involving
water and is recommended when cleaning exterior
surfaces prior to repainting.
Sandblasting: Finally--and undoubtedly most vehemently "not recommended"--sandblasting
painted exterior woodwork will indeed remove paint, but at the same
time can scar wooden elements beyond recognition. As with rotary wire
strippers, sandblasting erodes the soft porous fibers (spring wood)
faster than the hard, dense fibers (summer wood), leaving a pitted surface
with ridges and valleys. Sandblasting will also erode projecting areas
of carvings and moldings before it removes paint from concave areas
(see figure 12). Hence, this abrasive method is potentially the most
damaging of all possibilities, even if a contractor promises that blast
pressure can be controlled so that the paint is removed without harming
the historic exterior woodwork. (For Additional Information, See Preservation
Briefs 6, "Dangers of Abrasive Cleaning to Historic Buildings".)
Summary of Abrasive Methods (Mechanical),
Recommended: Orbital sander, belt sander (skilled operator only).
Applicable areas of building: Flat surfaces, i.e., siding, eaves,
doors, window sills.
For use on: Class II and Class III conditions.
Health/Safety factors: Take precautions against lead dust and eye
damage; dispose of lead paint residue properly.
Not Recommended: Rotary drill attachments, high pressure waterblasting,
sandblasting.
* Thermal Methods
Where exterior surface conditions have been identified that warrant
total paint removal such as peeling, cracking, or alligatoring, two
thermal devices--the electric heat plate and the electric heat gun--have
proven to be quite successful for use on different wooden elements of
the historic building. One thermal method--the blow torch--is not recommended
because it can scorch the wood or even burn the building down!
Recommended Thermal Methods
Electric heat plate: The electric heat plate (see figure 13) operates
between 500 and 800 degrees Fahrenheit (not hot enough to vaporize lead
paint), using about 15 amps of power. The plate is held close to the
painted exterior surface until the layers of paint begin to soften and
blister, then moved to an adjacent location on the wood while the softened
paint is scraped off with a putty knife (it should be noted that the
heat plate is most successful when the paint is very thick!). With practice,
the operator can successfully move the heat plate evenly across a flat
surface such as wooden siding or a window sill or door in a continuous
motion, thus lessening the risk of scorching the wood in an attempt
to reheat the edge of the paint sufficiently for effective removal.
Since the electric heat plate's coil is "red hot," extreme caution should
be taken to avoid igniting clothing or burning the skin. If an extension
cord is used, it should be a heavy-duty cord (with 3-prong grounded
plugs). A heat plate could overload a circuit or, even worse, cause
an electrical fire; therefore, it is recommended that this implement
be used with a single circuit and that a fire extinguisher always be
kept close at hand.
Electric heat gun: The electric heat gun (electric hot-air gun) looks
like a hand-held hairdryer with a heavy-duty metal case (see figure
14). It has an electrical resistance coil that typically heats between
500 and 750 degrees Fahrenheit and, again, uses about 15 amps of power
which requires a heavy-duty extension cord. There are some heat guns
that operate at higher temperatures but they should not be purchased
for removing old paint because of the danger of lead paint vapors. The
temperature is controlled by a vent on the side of the heat gun. When
the vent is closed, the heat increases. A fan forces a stream of hot
air against the painted woodwork, causing a blister to form. At that
point, the softened paint can be peeled back with a putty knife. It
can be used to best advantage when a paneled door was originally varnished,
then painted a number of times. In this case, the paint will come off
quite easily, often leaving an almost pristine varnished surface behind.
Like the heat plate, the heat gun works best on a heavy paint buildup.
(It is, however, not very successful on only one or two layers of paint
or on surfaces that have only been varnished. The varnish simply becomes
sticky and the wood scorches.)
Although the heat gun is heavier and more tiring to use than the heat
plate, it is particularly effective for removing paint from detail work
because the nozzle can be directed at curved and intricate surfaces.
Its use is thus more limited than the heat plate, and most successfully
used in conjunction with the heat plate. For example, it takes about
two to three hours to strip a paneled door with a heat gun, but if used
in combination with a heat plate for the large, flat area, the time
can usually be cut in half. Although a heat gun seldom scorches wood,
it can cause fires (like the blow torch) if aimed at the dusty cavity
between the exterior sheathing and siding and interior lath and plaster.
A fire may smolder for hours before flames break through to the surface.
Therefore, this thermal device is best suited for use on solid decorative
elements, such as molding, balusters, fretwork, or "gingerbread."
Not Recommended
Blow Torch: Blow torches, such as hand-held propane or butane torches,
were widely used in the past for paint removal because other thermal
devices were not available. With this technique, the flame is directed
toward the paint until it begins to bubble and loosen from the surface.
Then the paint is scraped off with a putty knife. Although this is a
relatively fast process, at temperatures between 3200 and 3800 degrees
Fahrenheit the open flame is not only capable of burning a careless
operator and causing severe damage to eyes or skin, it can easily scorch
or ignite the wood. The other fire hazard is more insidious. Most frame
buildings have an air space between the exterior sheathing and siding
and interior lath and plaster. This cavity usually has an accumulation
of dust which is also easily ignited by the open flame of a blow torch.
Finally, leadbase paints will vaporize at high temperatures, releasing
toxic fumes that can be unknowingly inhaled. Therefore, because both
the heat plate and the heat gun are generally safer to use--that is,
the risks are much more controllable--the blow torch should definitely
be avoided!
Summary of Thermal Methods
Recommended: Electric heat plate, electric heat gun.
Applicable areas of building: Electric heat plate--flat surfaces such
as siding, eaves, sash, sills, doors. Electric heat gun--solid decorative
molding, balusters, fretwork, or "gingerbread."
For use on: Class III conditions.
Health/Safety factors: Take precautions against eye damage and fire.
Dispose of lead paint residue properly.
Not Recommended: Blow torch.
* Chemical Methods
With the availability of effective thermal methods for total paint
removal, the need for chemical methods--in the context of preparing
historic exterior woodwork for repainting--becomes quite limited. Solvent-base
or caustic strippers may, however, play a supplemental role in a number
of situations, including:
* Removing paint residue from intricate decorative features, or in
cracks or hard to reach areas if a heat gun has not been completely
effective;
* Removing paint on window muntins because heat devices can easily
break the glass;
* Removing varnish on exterior doors after all layers of paint have
been removed by a heat plate/heat gun if the original varnish finish
is being restored;
* Removing paint from detachable wooden elements such as exterior
shutters, balusters, columns, and doors by dip stripping when other
methods are too laborious.
Recommended Chemical Methods
(Use With Extreme Caution)
Because all chemical paint removers can involve potential health and
safety hazards, no wholehearted recommendations can be made from that
standpoint. Commonly known as "paint removers" or "strippers," both
solvent-base or caustic products are commercially available that, when
poured, brushed, or sprayed on painted exterior woodwork are capable
of softening several layers of paint at a time so that the resulting
"sludge"--which should be remembered is nothing less than the sequence
of historic paint layers--can be removed with a putty knife. Detachable
wood elements such as exterior shutters can also be "dip-stripped."
Solvent-base Strippers: The formulas tend to vary, but generally consist
of combinations of organic solvents such as methylene chloride, isopropanol,
toluol, xylol, and methanol; thickeners such as methyl cellulose; and
various additives such as paraffin wax used to prevent the volatile
solvents from evaporating before they have time to soak through multiple
layers of paint. Thus, while some solvent-base strippers are quite thin
and therefore unsuitable for use on vertical surfaces, others, called
"semi-paste" strippers, are formulated for use on vertical surfaces
or the underside of horizontal surfaces.
However, whether liquid or semi-paste, there are two important points
to stress when using any solvent-base stripper: First, the vapors from
the organic chemicals can be highly toxic if inhaled; skin contact is
equally dangerous because the solvents can be absorbed; second, many
solvent-base strippers are flammable. Even though application out-of-doors
may somewhat mitigate health and safety hazards, a respirator with special
filters for organic solvents is recommended and, of course, solvent-base
strippers should never be used around open flames, lighted cigarettes,
or with steel wool around electrical outlets.
Although appearing to be the simplest for exterior use, a particular
type of solvent-base stripper needs to be mentioned here because it
can actually cause the most problems. Known as "water-rinsable," such
products have a high proportion of methylene chloride together with
emulsifiers. Although the dissolved paint can be rinsed off with water
with a minimum of scraping, this ultimately creates more of a problem
in cleaning up and properly disposing of the sludge. In addition, these
strippers can leave a gummy residue on the wood that requires removal
with solvents. Finally, water-rinsable strippers tend to raise the grain
of the wood more than regular strippers.
On balance, then, the regular strippers would seem to work just as
well for exterior purposes and are perhaps even better from the standpoint
of proper lead sludge disposal because they must be hand 'scraped as
opposed to rinsed off (a coffee-can with a wire stretched across the
top is one effective way to collect the sludge; when the putty knife
is run across the wire, the sludge simply falls into the can. Then,
when the can is filled, the wire is removed, the can capped, and the
lead paint sludge disposed of according to local health regulations).
Caustic strippers: Until the advent of solvent-base strippers, caustic
strippers were used exclusively when a chemical method was deemed appropriate
for total paint removal prior to repainting or refinishing. Now, it
is more difficult to find commercially prepared caustic solutions in
hardware and paint stores for homeowner use with the exception of lye
(caustic soda) because solvent-base strippers packaged in small quantities
tend to dominate the market.
Most commercial dip stripping companies, however, continue to use
variations of the caustic bath process because it is still the cheapest
method available for removing paint. Generally, dip stripping should
be left to professional companies because caustic solutions can dissolve
skin and permanently damage eyes as well as present serious disposal
problems in large quantities.
If exterior shutters or other detachable elements are being sent out
(6) for stripping in a caustic solution, it is wise to see samples of
the company's finished work. While some companies do a first-rate job,
others can leave a residue of paint in carvings and grooves. Wooden
elements may also be soaked too long so that the wood grain is raised
and roughened, requiring extensive hand sanding later. In addition,
assurances should be given by these companies that caustic paint removers
will be neutralized with a mild acid solution or at least thoroughly
rinsed with water after dipping (a caustic residue makes the wood feel
slippery). If this is not done, the lye residue will cause new paint
to fail.
Summary of Chemical Methods
Recommended, with extreme caution: Solvent-base strippers, caustic
strippers.
Applicable areas of buildings: decorative features, window muntins,
doors, exterior shutters, columns, balusters, and railings.
For use on: Class III Conditions.
Health/Safety factors: Take precautions against inhaling toxic vapors;
fire; eye damage; and chemical poisoning from skin contact. Dispose
of lead residue properly
General Paint Type Recommendations
Based on the assumption that the exterior wood has been painted with
oil paint many times in the past and the existing top coat is therefore
also an oil paint, (7) it is recommended that for CLASS I and CLASS
II paint surface conditions, a top coat of high quality oil paint be
applied when repainting. The reason for recommending oil rather than
latex paints is that a coat of latex paint applied directly over old
oil paint is more apt to fail. The considerations are twofold. First,
because oil paints continue to harden with age, the old surface is sensitive
to the added stress of shrinkage which occurs as a new coat of paint
dries. Oil paints shrink less upon drying than latex paints and thus
do not have as great a tendency to pull the old paint loose. Second,
when exterior oil paints age, the binder releases pigment particles,
causing a chalky surface. Although for best results, the chalk (or dirt,
etc.) should always be cleaned off prior to repainting, a coat of new
oil paint is more able to penetrate a chalky residue and adhere than
is latex paint. Therefore, unless it is possible to thoroughly clean
a heavily chalked surface, oil paints--on balance--give better adhesion.
If however, a latex top coat is going to be applied over several layers
of old oil paint, an oil primer should be applied first (the oil primer
creates a flat, porous surface to which the latex can adhere). After
the primer has thoroughly dried, a latex top coat may be applied. In
the long run, changing paint types is more time consuming and expensive.
An application of a new oil-type top coat on the old oil paint is, thus,
the preferred course of action.
If CLASS III conditions have necessitated total paint removal, there
are two options, both of which assure protection of the exterior wood:
(1) an oil primer may be applied followed by an oil-type top coat, preferably
by the same manufacturer; or (2) an oil primer may be applied followed
by a latex top coat, again using the same brand of paint. It should
also be noted that primers were never intended to withstand the effects
of weathering; therefore, the top coat should be applied as soon as
possible after the primer has dried.
CONCLUSION
The recommendations outlined in this Brief are cautious because at
present there is no completely safe and effective method of removing
old paint from exterior woodwork. This has necessarily eliminated descriptions
of several methods still in a developmental or experimental stage, which
can therefore neither be recommended nor precluded from future recommendation.
With the ever-increasing number of buildings being rehabilitated, however,
paint removal technology should be stimulated and, in consequence, existing
methods refined and new methods developed which will respect both the
historic wood and the health and safety of the operator.
NOTES
(1) General paint type recommendations will be made, but paint color
recommendations are beyond the scope of this Brief.
(2) Douglas R. Shier and William Hall, Analysis of Housing Data Collected
in a Lead-Based Paint Survey in Pittsburgh, Pennsylvania, Part 1. National
Bureau of Standards, InterReport 771250, May 1977.
(3) Any pigmented liquid, liquefiable, or mastic composition designed
for application to a substrate in a thin layer which is converted to
an opaque solid film after application. Paint and Coatings Dictionary,
1978. Federation of Societies for Coatings and Technology.
(4) For purposes of the Brief, this includes any area of painted exterior
woodwork displaying signs of peeling, cracking, or alligatoring to bare
wood. See descriptions of these and other paint surface conditions as
well as recommended treatments on pp. 510.
(5) See the Reading List for paint research and documentation information.
See also "The Secretary of the Interior's Standards for Historic Preservation
Projects with Guidelines for Applying the Standards" for recommended
approaches on paints and finishes within various types of project work
treatments.
(6) Marking the original location of the shutter by number (either
by stamping numbers into the end grain with metal numeral dies or cutting
numbers into the end with a pen knife) will minimize difficulties when
rehanging them.
(7) If the top coat is latex paint (when viewed by the naked eye or
preferably, with a magnifying glass, it looks like a series of tiny
craters) it may either be repainted with new latex paint or with oil
paint. Normal surface preparation should precede any repainting.
Reading List
Batcheler, Penelope Hartshorne, "Paint Color Research and Restoration."
Technical Leaflet 15. Nashville: American Association for State and
Local History (undated).
"Danger: Restoration May Be Hazardous to Your Health." The Old House
Journal. Vol. 4, No. 5 (May 1976), pp. 911.
Gola, Edward F. "Avoiding Mistakes in Exterior Painting." The Old
House Journal. Vol. 4, No. 6 (June 1976), pp. 1, 45.
"How to Assure a Satisfactory Paint Job." Scientific Section: Circular
784. Washington, DC: National Paint, Varnish and Lacquer Association
(undated).
Labine, Clem. "Selecting the Best Exterior Paint." The Old House Journal.
Vol. 4, No. 7 (July 1976), pp. 1, 1011.
Morton, W. Brown III and Hume, Gary L. The Secretary of the Interiors
Standards for Historic Preservation Projects with Guidelines for Applying
the Standards. Washington, DC: Department of Interior, 1979.
Paint Problem Solver. St. Louis: National Decorating Products Association,
1980.
"Special Issue: Exterior Painting." The Old House Journal. Vol. 4,
No. 4 (April 1981), pp. 7194.
Thorsen, John W. "Hazardous Waste: What is it? How to Handle it."
Professional Decorating & Coating Action. Vol. 43, No. 4 (September
1981), pp. 45.
-------------------------------
Special thanks go to Baird M. Smith, AIA (formerly Chief, Preservation
Technology Branch, TPS) for providing general direction in the development
of the manuscript. In addition, the following individuals are to be
thanked for their contributions as technical experts in the field: Royal
T. Brown, National Paint and Coatings Association, Washington, D.C.;
Dr. Judith E. Selwyn, Preservation Technology Associates, Boston, Massachusetts;
and Dennis R. Vacca, Pratt & Lambert Co., Carlstadt, New Jersey.
Finally, thanks go to several National Park Service staff members whose
valuable comments were incorporated into the text and who contributed
to the production of the brief: James A. Caufield, Anne E. Grimmer,
Jean E. Travers, David G. Battle, Sharon C. Park, AIA, Charles E. Fisher
III, Sara K. Blumenthal, and Martha A. Gutrick. Washington, D.C. September,
1982
Last Modified:
January 30, 1998
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