Illustration Courtesy of the Smithsonian Institution

By Robert M. Kelly

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 problems.

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.

Plaster Problems

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 a sandwich.

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.

Drywall Drawbacks

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 panels abutted.

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.

Paint Problems

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 persist.)

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 surface.

Robert M. Kelly is the principal at WRN Associates in Lee, Massachusetts (www.paperhangings.com).

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 for future generations. 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 longer.