Painting over wallpaper can be a huge time saver compared to the alternative of wallpaper removal. Before any finish coats of paint are applied, it is very important to prime the wallpaper with a coat of oil base or shellac base primer. Do not use a water base primer. A proper primer coat will create a barrier preventing the new paint from liquefying the wallpaper adhesive, which would in turn cause loosening of the wallpaper.
Painting over wallpaper requires a primer coat. The first step is to cut-in the primer.
Pour a quart or so of oil base or shellac base primer into a clean 1 gallon bucket, for cutting in.
Select a 2, 2½, or 3 inch angular or straight paint brush. These are good sizes for cutting in a wall. Use a China bristle paint brush for oil base primer or a polyester paint brush for shellac primer.
The primer should be cut-in anywhere that the roller cannot reach. These areas include the following:
If you choose to protect all the adjacent surfaces at areas of cut-in with a layer of safe release masking tape, try to prevent excessive amounts of primer from getting onto the tape. Masking tape tends to allow small amounts of paint to seep onto the protected surfaces it is meant to protect.
When painting over wallpaper, the second step is to apply the primer to the larger areas with a roller.
For a small to medium size job, a roller tray is a convenient set up to roll primer. If you use a metal roller tray, a disposable plastic tray liner will save you time cleaning up. Pour about ½ gallon of primer into the tray leaving enough space to prepare the roller with paint. For large jobs a 5 gallon bucket and roller grid is more convenient. Place a 5 gallon roller grid into the container, with the grid hooks hanging from the rim of the bucket. Pour up to 2 ½ gallons of primer into a 5 gallon bucket.
Slide a ¼ to ½ inch synthetic roller cover onto a 9 in. roller frame, with the beveled edge facing away from the handle. Attach a 2-4 ft. fiberglass extension pole to the end of the roller handle. This size works well with a standard height wall. A 4 ft. wooden extension pole will also suffice. A 4-8 ft. extension pole works better for walls over 10 ft., and may also be preferable for ceilings.
With the extension pole attached to the roller frame, dip the roller into the paint. Roll it several times over the shallow section of the roller tray or bucket grid until the primer is evenly distributed over the roller.
Place the loaded roller midlevel on the wall and roll in an up and down motion. Spread the primer evenly to the top and bottom of the wall making a swath 12 to 18 inches wide. Repeat the process until the wall is completely covered.
Before painting over wallpaper with finish coats, some prep may be necessary. Be sure to allow the primer to dry before beginning any prep
Textured wallpaper presents a problem as far as preparation is concerned. Applying joint compound over the seams or any other problem areas may interrupt the textured pattern and leave unsightly smooth spots. Limited areas of drywall spackling may be the best solution for prepping textured wallpaper. Assess your situation to determine the best method of preparation.
Wallpaper seams on smooth wallpaper often leave small ridges. This is more noticeable after the wallpaper has been primed. In some cases the wallpaper may be a little loose around the seams. Prepping the seams will address these issues and improve the appearance of the wall.
Carefully shave any visible ridges on the wallpaper seams with a razor blade scraping tool. Cut away any loose wallpaper around the seams with a utility knife. Also inspect the entire wall for any loose areas of wallpaper and cut away with the utility knife.
Drywall Joint Compound
Pour a little dry mix (hot mud) drywall joint compound into a mud pan. The 30 minute joint compound is convenient for small projects because of its faster dry time. Add a small amount of water and mix thoroughly until the joint compound paste has a smooth consistency.
Spread a thin layer of the drywall joint compound over each wallpaper seam using a 6 inch flexible joint knife. Pull the edges of the mud tight with the joint knife. A thin layer of joint compound is very important in order to avoid creating a hump at the seams. The layer of drywall joint compound should be about 6 inches wide extending about 3 inches from each side of the seam.
Apply a thin layer of drywall joint compound over any additional areas where loose wallpaper has been cut away.
When the drywall joint compound is dry, sand it smooth with a medium/course or a medium sanding sponge. You should end up with a completely flat surface. The edges of the joint compound should not be visible. If they are visible after the initial sanding, additional sanding will be needed.
Spot prime all areas covered with drywall joint compound. Either the same primer used to seal the wallpaper or an acrylic primer is suitable as a spot primer for the joint compound.
If other problems exist, such as holes and gouges, additional prep should be performed before painting over wallpaper. Drywall Spackling is an easy fix for these types of problems.
Painting Finish Coats
After the priming and the prep has been completed, painting over wallpaper is much the same as painting interior walls. Typical water base interior wall paints can be used.