If you're planning to hang new wallpaper, the first problem you often face is what to do about the old wallpaper.
As a general rule, it's best to remove old wall covering. Always remove it if there are more than two layers because the old paper is likely to pull away from the wall if you paper over it. And you also should always remove a vinyl, foil or textured covering before putting up a new one.You don't have to remove other old wall covering, however, if there's only one layer and it's still adhering tightly. Test the old wallpaper by running your fingertips over it. If you hear a crackling noise, the paper is loose and should be removed. Also check the edges and corners by trying to pry them up with a putty knife. If large sections lift off, it's a sign that the covering needs to be removed.
If the old covering passes the crackle and corner tests, glue down any loose edges with white glue and wallpaper paste. Then wash the surface with detergent or a mild solution of household bleach and water. Let the wall dry thoroughly, then coat it with wallpaper sizing, a glue designed to be a base for wall coverings, or with an alkyd sealer made as an undercoat for wall coverings.
If you're going to remove an old covering that's stripable, pry up a corner and pull it down to peel it off. Most vinyl coverings strip off easily.
If the covering is stubborn, it must be soaked - a messy job. Use old sheets and bedspreads as dropcloths.
Before soaking, abrade the wall covering's surface with coarse sandpaper, a wire brush or a scoring tool made for the purpose. Take care not to gouge the wall as you work.
Mix chemical wallpaper remover according to the manufacturer's directions. Wear rubber gloves and goggles and brush or sponge several coats of the remover to the wallpaper. Let it soak as directed, then scrape the paper off with a wide-blade taping knife.
A mixture of 1 part vinegar and 10 parts water also makes a good paste softener. The smell, while nontoxic, may be irritating, so ventilate the room well.
Brushing or sponging on remover is a slow process. You can speed things up considerably if you put the remover in a pressurized garden sprayer. Before you fill the sprayer, make sure that it's free of any insecticide residue. Hold the nozzle a couple of feet away, and spray it liberally on the wall until the paper is wet but not dripping. Allow the remover to soak in, and the paper should almost fall off the wall. Spray any resistant areas again.
Use a window squeegee with very hot water to remove old, wet wallpaper paste from plaster walls. Wash newly stripped walls with clean hot water and a little household bleach (one-quarter-cup bleach to 2 gallons water). Before putting up the new covering, let the walls dry thoroughly - a few hours at the least, possibly several days if it's humid.