Painting over wallpaper is not advised by professionals and should be done only as a last resort. If possible, it is preferable to remove the old wallpaper before painting. The removal of wallpaper can be very messy and time-consuming, and may require strong chemical sprays or wallpaper glue strippers. However, the preparation needed to paint over the wallpaper is also a long and tedious job, and it may be worth the trouble of removing the old wallpaper since so much time and effort is invested in preparing the wall to be painted. However, if the wallpaper is already painted over with one or more layers of paint or the wallpaper was installed over multiple layers of wallpaper, removing the wallpaper becomes too difficult and the wall may have to be painted over the wallpaper.
The Option to Remove the Wallpaper
Since removal of the wallpaper is preferable to painting over the wallpaper, mention of this step is mandatory in this article. Removal can be as easy as loosening a corner of the wallpaper and pulling it off the wall. If the paper backing of vinyl wallpaper remains after the vinyl surface is removed, this paper needs to be softened with water and scraped off with a putty knife. If the wallpaper does not come off that easily, it needs to be scored with a utility knife or a Paper Tiger, which is a hand-held tool which is rubbed over the surface of the wallpaper. Scoring the wallpaper perforates the wallpaper so that a wallpaper paste dissolving chemical can be sprayed on top of the wallpaper and soak through the holes to dissolve the wallpaper paste below it. The chemical is given some time to soften the glue, then the wallpaper is scraped off with a putty knife, and the chemical and dissolved glue are washed off. Alternatively, a wallpaper steamer can be applied to the wallpaper like a hot iron, and the wallpaper scraped off a patch at a time.
The Option to Paint Over the Wallpaper
If it is decided to paint over the wallpaper after all, the wall must still be cleaned and prepared before painting. Peel off any loose pieces of wallpaper. Seams in the wallpaper that have come loose may have to be glued down with wallpaper paste. Repair any defects in the wall or in the wallpaper by sanding. It is important to have as smooth and flat a surface as possible.
Priming the Wallpaper
The wallpaper will have to be primed with an oil based, stain killing primer. Alcohol based primer or fast drying shellac may be able to be used instead, but do not use a water based primer on wallpaper as this may soften the glue and cause the wallpaper to loosen and peel off after it is painted. If the wallpaper is made of vinyl instead of paper, use an acrylic primer instead of an oil-based primer. Be sure to have adequate ventilation in the room while working with oil-based primer, as the fumes can be quite obnoxious. This priming must be done before the next step to keep the wallpaper paste from softening and the old wallpaper from peeling any further. Use a short nap paint roller to apply the primer.
Leveling the Wallpaper
After the first coat of primer is dry, file down all seams with a file. It may be necessary to level the walls with drywall mud and a wide drywall knife. Cover any seams and places where there is an uneven area in the surface, for example where the old pieces of loose wallpaper have been torn off. Try to make the wall surface as smooth and flawless as possible. This is especially important if the wallpaper is textured, because the texture will show through the paint unless completely covered by the drywall mud. After the drywall mud is dry, sanding may be necessary to provide a completely flat, smooth surface for painting. If the surface is still not smooth enough, texturing the wall might be considered.
Painting the Wallpaper
Apply one or more coats of primer on top of the sanded drywall. After the last coat of primer is applied, use clear or white caulk to seal the edges of the old wallpaper at the ceiling line, the baseboards, and around any window or door trim. This is not necessary in the corners of the room, as it is only necessary to prevent the edges of the old wallpaper from loosening under the paint. When this is dry, one or more coats of paint are applied to the wall.