Some Important Preparations Before Wallpapering a Room

Hanging paper on a wall is as easy as painting it (although some rooms, like the bathroom or kitchen, may be more difficult to cover than a bedroom or living room). The key to a successful papering job lies in preparing the walls properly. The walls will fall into one of three categories: painted walls, new walls or papered walls.

Walls painted with glossy enamel should be deglossed before papering otherwise the paper will not adhere to the surface. There are commercial solvents (one brand is Liquid Sandpaper) that will roughen the slick surface and provide the proper tooth needed for good adhesion. Sometimes washing the walls with a heavy-duty detergent will work as well. Solvents and heavy-duty detergents are harsh chemicals, so wear rubber gloves and allow fresh air to circulate in the room.

Newly installed drywall should be be primed before papering. Priming creates a smooth surface for the paper and seals the surface so the paste will not penetrate into the wall material. A coat of oil-base wall primer is probably the most popular sealer. You can also use a special glue size called wallpaper primer/sealer to prime the walls. Either of these primers can be applied, like paint, with a brush or roller.

Older houses will usually have plaster walls. These can be primed with glue size. Because plaster is mixed with lime, it has a high alkaline content. This alkalinity can stain the wallpaper or keep it from sticking to the wall. Usually the alkalinity fades with time, and the wallpaper can be applied to the primed plaster wall. In some cases the glue size may reveal areas that still have a high content of alkali. These are called ''hot spots,'' and they show up as colored patches -- pink, red or purple -- when the wall is sized.

Hot spots must be treated before you can paper over them. You can seal the alkali into the wall by brushing a coat of shellac over the area. This is easy enough, but it's possible that in time the alkali can bleed through the shellac and discolor the wallpaper. A more effective treatment involves neutralizing the alkali with a solution of 28 percent acetic acid. Mix one part acid to two parts water. Wear rubber gloves and swab the solution over the spot. When the color disappears, the alkali is gone. You can also use ordinary household vinegar for this process, but it takes longer to accomplish. After the alkali is neutralized, allow time for the spot to dry then finish by resizing.

If your walls are covered with paper, you must decide whether to apply new paper directly over the old or remove the existing paper. Generally it's best to remove the old paper; otherwise the patterns and seams from the old may show through the new. In some cases new adhesives may react with the old glue causing both layers to come undone.There are times, however, when it's best to leave the old wallpaper in place. Foil papers, for example, are often difficult to remove because water or steam cannot penetrate through the covering to soften the adhesive beneath. If the paper is intact with flat seams, then smooth spackling compound over the seams and prime with latex primer. You may need two coats of primer to cover dark paper.

Paper that was applied directly to unprimed wallboard may be difficult or impossible to remove without pulling off chunks of the wallboard itself. If the paper is securely bonded to the wallboard, it's probably best to seal the seams, prime, then paper over it.

Some vinyl or mylar papers are called strippable wall coverings and were designed for easy removal. To remove them, pry up a corner with a utility knife and carefully pull down from the wall. Other strippable papers require a little soaking before you can peel them off. With older wallpapers you have to apply enough moisture to penetrate through the paper to soak the glue holding it in place. This is done by applying warm water or commercial wallpaper remover with large sponge. Multiple layers or papers that adhere tightly to the wall should be perforated to allow the water to penetrate. There are a number of ways to pierce the paper; you can slash it with a utility knife, drag a handsaw across the surface or abrade it with coarse sandpaper. You can also buy a wallpaper perforation tool at paint stores and home centers. Be careful not to damage the wall behind the paper when cutting.