The first step is to repair any defects in the wall surface. With drywall, any dents, nail holes, etc., should be filled with drywall patching compound. After the proper drying time for the particular compound you are using, the repaired areas should be sanded to leave a smooth, level surface. With plaster walls, any defects should be filled with a gypsum based filler. As the filler dries, it draws together slightly, and after around 30 minutes you should go over the area again with some more filler. Larger defects have to be reinforced with joint tape. The crack should be scraped clean and moistened with water. Fill the crack with filler and the place the joint tape (very thin fiberglass tape) over the crack in a vertical direction. Now, go over the tape with a thin layer of filler and then carefully smooth the filler using the spatula. When the area is dry, you can carefully smooth it with sandpaper.
Once your walls have been repaired, they should be cleaned of any sanding residue, grease or obvious stains. Stains may be spot treated with stain killing primer as necessary. Any moldy areas must be treated with a bleach solution prior to application of any stain killing primer. Especially difficult stains may require the use of oil-based primers. It is normally recommended to cover these treated areas with an acrylic primer. With the exception of primers that specifically call for sanding of any gloss finish, you are ready for priming.
Preparation for wallpaper
Experienced wallpaper installers have used different products with outstanding results other than those listed. These guidelines were developed as a starting point for the do it yourself homeowner and are considered the best guess as to what type of product will suit your needs. This information is based on subjective, collective information from several wallcovering professionals and may not suit your particular application. It is imperative that you fully understand all safety precautions when dealing with these products, and always follow the manufacturer's instructions completely. You must also follow any instructions from the wallpaper and adhesive manufacturer as to what type of surface preparations are needed for effective bonding.
On drywall (painted with latex or oil) use an acrylic primer or prep coat.
On drywall (painted with builder's flat) use a primer/sealer.
On new plaster (after the proper curing time) use size mixed with the adhesive you plan to install the final wallcovering with to prepare the surface.
On older plaster (painted with latex or oil) use an acrylic primer or primer/sealer.
On any wall with suspected defects or damage use a primer/sealer.
On any wall with stains or mold issues use a stain killer primer. (mold must be treated with a bleach solution first)
On new or repaired drywall use a primer/sealer.
On a wall surface with residual wallpaper paste use a primer/sealer.
On decorative borders applied to a painted surface refer to wall type mentioned above.
Drywall: Also known as sheetrock, wallboard, or gypboard. The biggest part of the gypsum rock used in this country goes into wallboard for homes. Drywall is formed by sandwiching a core of wet plaster between two sheets of heavy paper. When the core sets and is dried, the sandwich becomes a strong, rigid, fire-resistant building material. Fire-resistant because in its natural state, gypsum contains water, and when exposed to heat or flame, this water is released as steam, retarding heat transfer. Drywall is simply cut, butted together, nailed to the wall studs, and all seam areas and nail holes are finished with joint compound creating a smooth uniform appearance.
Plaster walls: Before 1900, lime-based plaster was used. It was mixed with animal hair and sand to give it stability and strength. After 1900, a gypsum-based plaster was used. A three-coat system was used in either case. First a scratch coat, which was pressed into the lath to form the plaster "keys" to hold it in place. Lath boards are the series of boards nailed to the wall frame spaced 1/4" apart. This spacing allows the scratch coat to mushroom between the gaps and anchor the plaster in place. This is followed by what is known as the brown coat, which is the first step in creating a level surface. The finish coat is toweled under pressure until the surface is mirror smooth. Lath boards are usually wood, though they can be metal in commercial applications due to increased fire resistance. Buttonboard plaster came into use in the 1950s. Buttonboard is a 3/4" thick material similar to drywall with numerous holes in it to serve the same purpose as the spaces between the lath boards. Only two coats of plaster are applied to buttonboard. The last type of plaster is "blueboard" plaster. This is drywall with a blue facing paper. The drywall is installed as normal and then one skim coat of plaster is applied to the entire surface.
How to tell the difference: The easiest way is to remove an electrical outlet cover, if the wall is more than 5/8" thick, it is plaster. Most modern houses are drywall though there are exceptions.
Primer: Most primers are applied to make the substrate more uniform for acceptance of the finish coat. They also improve the adhesion of the topcoat. Not all primers will allow the wallpaper to slide easily on them during the installation process. They also will improve the removability of wallpaper and decrease the chances of wall damage. These can be either water based (acrylic) or oil based. All paint companies manufacture primers.
Primer/Sealers: Also known as DRC, drywall repair clears. Can provide the best insurance on a good installation. It is a special penetrating primer that is designed to penetrate the wall surface and seal up any problem areas due to wall damage or any situation where wall surface anomalies are suspected. These products are available in several mixtures to address specific needs. A colored (pigmented) acrylic primer/sealer is the most common because it can be used on all surfaces. It's water based, easy to clean and the coloring helps prevent any discolorations from showing through the paper. These products protect the underlying drywall, provide a good surface for adhesion, and increase the slip of wallpaper. Examples of primer/sealers are Scotch Paint's Draw-Tite, Zinsser's Gardz, Roman's Liquid Drywall, and Sherwin Williams' PrepRite Drywall Conditioner.
Prep Coat: Acrylic primer that normally, when dry, leaves a tacky surface. This surface allows wallcoverings to easily adhere to the surface. Sometimes referred to as a primer/size. Examples of prep coats are Roman's R-35, Zinsser's Z-54, California Paint's Prep 'n Size, Golden Harvest's BITE, Muralo's Adhesium, Duron's Tack Prep, and Benjamin Moore's Wall-Grip.
Stain Killer Primers: Should be used for walls with problematic stains such as grease, recurring mold, etc. They prevent these types of stains from bleeding through the wallpaper. This product would be used to spot-treat these areas or as a total primer base. These primers are also excellent for covering brightly painted surfaces that may otherwise bleed through the final wallcovering. Most stain killer formulas contain anti-microbal agents to prevent future growth of any type of mold; however, existing mold must be removed using a 3:1 water to bleach solution prior to application of the primer. All paint companies manufacture stain killing primers.
Size: In the case of plaster walls, it will prevent too much paste from being absorbed into the wall. It's use on drywall applications is not so much to prepare the wall, but to provide added adhesion for the final installation of wallcovering. It usually comes in the form of a white powder that is mixed with water according to the manufacturer's instructions. Another form of size is to coat the walls with a thinned down version of the adhesive that ultimately be used in the installation of the wallcovering. Many wallpaper manufacturers specifically request it's usage on any wall type though it is traditionally associated with plaster walls. All wallpaper adhesive companies manufacture size.