Understanding wall surfaces and how to prepare them for paper.
Intrepid old-house owners often encounter some puzzling, flaky, and
freaky surfaces after stripping their wallpaper:
| Illustration Courtesy of the
“I found some white, powdery stuff that comes off with a sponge.”
“Under the last coat of wallpaper was something brown, sticky,
and sort of gross.”
“I think it’s drywall, but it’s soft and brown!
And the joints look like they have putty in them!"
“I have plaster, but it’s sort of crumbly and sandy.
Should I paint it before papering?”
Whether your walls are 18th-century lime plaster or 1950s drywall,
the question after stripping is: What do I do now? This wall-prep
primer will give you some insights into past paper-hanging practices
as well as help you understand and overcome your present wall-prep
How Wallpaper Works
For all its ability to convey style in shorthand, wallpaper is nothing
more than ink on paper, making it abundant and fairly inexpensive
for the last 250 years. Even so, hanging wallpaper can be problematic.
One major reason is that wallpaper expands and contracts up to 1/4"
during the pasting and drying process. In its moist, expanded, fabric-like
state, paper can more easily follow the roll of the wall, but when
dry, the paper contracts, putting stress on the wall surface.
This contraction is the reason why wall- prep for paper is different
than for paint. Unfinished walls, whether wood, plaster, or drywall,
are porous, but if a wall is too porous the paste may get lost in
the wall, leaving few solids to anchor the paper. If a wall is too
sealed, it may not have enough texture for the paste to hang onto,
and the paper may curl up after drying. The semiporous wall—one
that is receptive to adhesive—is the best surface for wallpaper.
What’s needed is a sound “wallpaper sandwich”—wall
and wallpaper united by a thin layer of high-moisture adhesive.
Porous walls can be a sized to reduce porosity, while sealed walls
can be coated with an acrylic to promote adhesion.
Because traditional lime-plaster walls dried hard, they made an
ideal surface for papering in many ways, but they had their pitfalls.
Concentrations of lime (known as hot spots) could burn wallpaper
colors with excess alkalinity. Paperhangers could neutralize the
lime with vinegar or a solution of zinc sulphate. However, the same
color variations could also be caused by suction spots, which could
occur anytime the porosity varied. The danger of hot spots faded
as lime plaster gave way to gypsum plaster.
Hard as they were, plaster walls were also porous and needed to
be tempered with glue size (a liquid that reduces the absorption
of an adhesive). Once sized, the wall could easily accept the paste
used on the wallpaper. The paste would evaporate into the wall,
leaving a stable, easily removable decoration. Although paper could
be removed, it often wasn’t. This resulted, after several
generations, in something that more resembled a layer cake than
Past Solutions In the past the best trade
practice for prepping walls called for a slow-drying coat of lead
paint, but shortcuts abounded. Oil paint often covered this coat.
When homeowners tried to paper over these oil-painted walls, the
wall was hard and impervious to moisture. It could be prepared for
papering by washing with sal (salt) soda and sanding to abrade the
surface, providing a key for the wallpaper paste.
Calcimine and whitewashed plaster walls, by far the most prevalent,
were even more difficult to prepare. These water-based materials
made with simple chalk or lime “pigments” incorporated
little binder. They couldn’t be papered over without the contraction
of the paper lifting them from the wall and ruining the job. A rosin
varnish coat often lay underneath. It was so slick that paper wouldn’t
easily stick to it. These coats, too, were often washed and sanded,
although the most rigorous advice was to remove the varnish with
strong chemicals. Glue size was used for such slick walls, since
it promoted adhesion, but it wasn’t quite enough in many cases.
This is why sticky additives like molasses, sugar, and syrup were
common. They helped wallpaper cling to varnished or painted surfaces,
but may also be responsible for many a weary old-house owner’s
complaint about stripping wallpaper.
Today’s Fixes As in the past, the
best way for old-house restorers to prep plaster walls is to size
them. The first modern wall-prep product came along in the early
1970s. It was an acrylic wall primer called “Wallpaper Prep-Coat”
from Swing, Ltd., a Canadian company, and based on a commercial
version of acrylic first produced by the Rohm & Haas Company
in 1927. The flexible acrylic film was effective in sealing most
surfaces and yet promoted adhesion. An important additive was diatomaceous
earth, which is a fancy way of saying silica, or, even simpler,
grit. (Silica is the inert transparent additive that turns gloss
finishes to matte.)
In this modern “size,” the grit provided tooth for the
wallpaper to hang onto. It was quickly joined by competing products.
The original “Wallpaper Prep-Coat” is still in production,
but the generic term “prep coat” has since come to mean
any acrylic wall-prep product. After walls are stripped, you should
apply the acrylic prep coat with a brush, paint pad, or roller.
The standard coverage is 600 square feet per gallon, much more than
the 400 square feet standard for paint. But prep coats are often
overspread because they are so thin. Spread a full coat, paying
particular attention to where the trim paint overlaps the wall—the
most likely place for wallpaper adhesion problems. If the prep coat
beads up on an oily or freshly painted surface, you may need to
lightly sand before application.
The first attempts at a “dry” alternative to
wet plaster walls, around 1900, were semirigid composition materials
of felt, fiber, and animal hair. By 1904 early drywall composed
of gypsum base surrounded by cardboard was available, but this wall
material created a new problem—uneven joints where the drywall
The 1922 Painter and Decorating Contractors of America manual’s
chapter on “wall-board” advised that “As a rule,
cracks between sections...are covered with mouldings put on by the
carpenter...if no mouldings are to be used, it is very difficult
to fill the cracks with putty so they will not show.” But
by 1949, the manual had much happier news: “Special joint
cement and perforated joint tape have been perfected.”
Past Solutions This period of experimentation,
especially around 1930, explains the odd-looking joints uncovered
by old-house owners. The earliest fillers were based on materials
from the painter’s shop: linseed oil or varnish, chalk, and
lead already in use for plaster repairs. Plaster of Paris was widely
used for the base repair, but it could powder, shrink, and fall
out. “Swedish putty” or spachtel, a more tenacious substance
that dried harder, became a standard final coat for repairs.
Another problem arose when wallpaper was hung on drywall. The wallpaper
would often become laminated to the top layer of the cardboard face,
nevermore to be separated from it. Anyone who has been part of a
wallpaper stripping job of this type knows how tenaciously the two
layers of paper will bond together.
Cheap varnishes were also used to seal drywall, as they had plaster.
This explains why it is common to run across shiny, clear surfaces
when stripping wallpaper from old drywall and plaster. A new reason
for sealing drywall became obvious after the first few attempts
were made to strip wallpaper from it.
Today’s Fixes Premixed joint compound
is the present day answer to most patching needs, but because it
shrinks, it may need multiple applications. Joint compound has become
so prevalent that it is sometimes confused—even by historic-
house curators—with plaster. But joint compound, unlike plaster
or Spackle, remains soft after drying and can rewet during hanging
or stripping of the wallpaper. It’s best to seal it with an
oil or what is called a drywall repair clear (DRC).
Despite the present-day success of drywall, it requires care before
papering. When finishing the joints, you may want to use a sponge
rather than sandpaper to feather the edges of the joint compound.
Sanding raises dust and also tears into the facing paper. Subsequent
coatings can “raise the grain,” which may result in
roughness and a need for additional sanding.
Drywall should be sealed prior to paperhanging. The better quality
prep coats approach oil in sealing properties, but the gold standard
for sealing drywall remains oil-based paint. Due to VOC (volatile
organic compound) regulations that aim to reduce pollutants, many
oil-based paints have been completely reformulated. Years ago, an
alkyd primer could usually be papered over successfully, but that
is no longer a safe bet. To ensure adhesion, it is necessary to
apply a prep coat over alkyds. For isolating a suspect surface (for
example, dusty, stained walls with chipped or scaling surfaces)
there is no substitute for a slow-drying oil-based paint.
What if your walls have only a few coats of a reasonably good latex
or acrylic paint? Do you really need additional wall prep?
Acrylic paints have been improving by leaps and bounds. Many acrylic
paints can withstand the stress of a paperhanging project. Years
ago the flat paints were suspect, but now a top-quality flat can
be as good as an eggshell or semigloss at holding out moisture and
protecting the wall. But there is still a large consumer market
for inferior paint at discount prices. The painting public is inclined
to skimp on paint specifications, especially when covering thousands
of square feet.
Today’s Fixes Unless you’re
sure of the history of the wall and the quality of the paint, the
safest course is to apply an acrylic wallcovering primer to all
existing paint prior to papering. Even oil-based paints, which are
naturally more resistant to moisture, may benefit from prep coats,
because of the enhanced adhesion. One can of wallpaper primer (about
$15) is cheap insurance if it keeps the paper on the wall.
Multiple Paint Coats Wall prep for the
outer layer may be beyond reproach, but what about the weak layer
of paint, the one that is four or five layers down? The chain of
paint layers is only as strong as its weakest link, and many wallpaper
jobs have been ruined by the wallpaper pulling up this hidden problem.
Today’s Fixes Sometimes a professional
may be able to test the surface, or suggest a lower moisture paste
that pulls less on the paint film. Sometimes liner paper can serve
as a sacrificial layer to test the surface. (If the liner ruptures
in a few areas, repairs can be made. But even then, problems may
If it seems likely that a weak paint film may sabotage the project—based
on how similar paint layers in similar rooms have reacted, for example—then
paper may not be a viable choice. Or, a new surface may need to
be created, for example, by installing a new plaster or drywall
Robert M. Kelly is the principal at WRN Associates in Lee, Massachusetts
More on Sizing
For old-time paperhangers, who were never without their
glue pots, size was very close to a universal solution
for plaster. Most size was granular, soaked in water,
and then heated to form a liquid. Ready-mixed glue size
came to retailer’s shelves by the 1920s and such
versions as Sisk’s and Adhesium were heavily advertised.
Glue size was great for plaster walls, somewhat less effective
on oil-painted walls, and a disaster when used over weakly
bound paints like calcimine and whitewash. Similarly,
glue size by itself could not cope with the new challenge
of drywall, or with the weakly bound latex paints that
came into use after World War II.
Inexpensive and easy-clean-up latex paints were effective
at covering vast amounts of drywall. But when these walls
were papered, failures were common. Often, the top layers
of drywall would adhere to the back of the paper. If the
wallpaper did not pop off after the wallpaper dried, creating
a daunting repair, it simply lay in wait. When it came
time for removal, there was hell to pay.
Today’s prep-coats are often called “sizes,”
an echo of the product they replaced. But they do not
size—they seal. (And yet, they do not produce a
monolithic film, but one perforated by micro-holes. This
allows the high-moisture pastes still in use to evaporate
into the wall.)
There are many varieties of prep-coats. Some are for priming
vinyl wallcoverings. Some types are pigmented, so that
a white, nearly opaque film results. Many of them can
be tinted. What sets prep- coats apart from other primers
is that they’re specifically formulated for wallcovering.
While there are some all-purpose types, these are pigmented
and they have a significant drawback for the historic
homeowner. The pigmentation buries historic plaster under
an opaque film. The translucent types offer just as much
adhesion promotion, and yet leave the plaster visible
Shredded drywall, a byproduct of stripping problems, became
so widespread that a special type of primer, also based
on acrylics, was developed to seal it. These products
are called drywall repair clears (DRCs) and are similar
to prep coats, but they offer more sealing properties
and less adhesion promotion.
More on Patching
Today there’s a wide variety of patching materials
available. The old-time Swedish putty (spachtel) types
are still around. They’re now known as “spackling
compounds” and are often sold premixed. They shrink
little and dry hard. Plaster of Paris is still an option,
though it has not gotten any easier to handle. The powdering,
high-shrinkage, and porosity remain, so this product is
mainly for professional use.
Some joint compounds are called “topping compounds”
and are intended for the last coat before painting. These
are smoother but more brittle than the all-purpose types
and are adequate for paperhanging.
Joint compound takes a few hours to dry but there are
quicker alternatives. One is a so-called “hot mud,”
supplied in powder form that can dry rock hard in a matter
of minutes. These types of dry mixes are recognized by
their “-90,” “-45,” or “-20”
suffixes. They’re called “hot” because
the chemical reaction responsible for the quick drying
makes them warm to the touch. Hot muds need to be sealed
well and prep-coats may not be up to the job. Again, to
seal use hot mud with DRC or an oil.
Recently, lightweight spackling compounds have arrived
for small repairs. It’s easy to identify them by
picking up the plastic pail. If it’s unexpectedly
light, you have a lightweight patching product. They use
a newer technology (micro-glass beads) and dry quickly,
without shrinking. They may need a little water to make
them more easily spreadable.
A Few tips:
Stripping: Most all wallpaper can be removed with enough
soaking. If you come across genuine historic paper,
or decorative painting, stop and seek professional help.
Both can be damaged by excessive moisture.
Plaster: If it has never been painted, it can still
be glue-sized. It can also be coated with a translucent
prep-coat. Don’t coat with an opaque primer; this
buries the history of the wall, which may include evidence
of alterations such as former doors or windows.
Patching: For best results, use plaster for plaster
repairs and seal joint compound, spackling compounds,
and hot muds.
Prep-coats: Paint primers and wallpaper primers are
not the same. Read the labels.
Lining paper: Lining paper is a worthy upgrade for
costly papers. Like underlayments for carpet, or linings
for drapes, it makes the wallpaper look better and last