Drywall paper can be damaged when a wall is gouged while moving furniture or other common household activities. Water can also damage drywall paper causing the surface to pucker and ripple. This can happen when removing wallpaper with a liquid stripper.
The most important part of repairing damage like this is sealing it so it doesn't absorb any moisture from the repair compound. If you don't seal the surface, the repair process will cause the undamaged layers of paper to bubble and ripple and no amount of sanding will fix the problem. Use the following procedure for a trouble free repair of torn drywall.
Cut all damaged paper away from the gypsum core. If only the first few layers are damaged, cut down to the first undamaged layer. Cut cleanly around the perimeter with a sharp drywall knife or a razor blade. Tilt the blade out a little to cut at a slight angle, completely through the paper. Gently peel the rough fringes of loose paper free, leaving a solid layer behind.
Coat the exposed fibers, edges and surrounding wall with oil or shellac primer-sealer. Stir the sealer or shake the can first to be sure it's mixed well and apply two, thick coats with a brush. Let the primer dry for about an hour between coats and let the final coat dry completely before proceeding.
When the second coat of sealer is dry, use a joint knife to smooth out the raised paper fibers. Go over the surface once or twice with the blade in a downward stroke to flatten them out.
Mix some setting-type joint compound and use it to cover the damage. Apply a skim coat over the whole area holding the joint knife at an angle to "butter" the surface with back and forth strokes. Leave about 1|8in. of mud over the damage and the surrounding drywall.
Immediately skim the excess mud from the surface to leave a thin coat that begins to cover the rough paper. Hold the knife at a 30° angle and press down hard, but not hard enough to dig into the surface, as you skim from one side to the other without stopping. Scrape the collected mud from the knife onto the rim of a mud pan and finish skimming in parallel strokes. Clean the knife off after each stroke to keep the mud under control.
Ignore any ridges left between the rows by the edges of the knife blade, these will be removed after the mud has dried. Around the perimeter of the patch, skim the mud as thin as possible to help blend it into the surrounding wall. Let the mud set for about 30 minutes or until it has hardened.
When the mud is dry, scrape off any ridges using the joint knife to cut them off with upward strokes. If necessary, use medium grit sandpaper to lightly sand the whole patch to smooth out any other bumps or protrusions. At the edges of the patch, sand to feather it with the surrounding surface.
Wipe the sanding dust off with a damp rag and apply a second coat of mud, skimming in the opposite direction from the first coat. If you skimmed the first coat vertically, apply the next with horizontal strokes. This will help to level out an irregular surface.
Let the mud dry and if necessary, apply another coat to build up a patch that hides the damage and smoothes out the wall. As before, scrape off any ridges first and sand if necessary, before skimming the patch again. Stroke with the knife in the opposite direction from the previous application.
Let the final coat of mud dry well and then sand it using light pressure in wide back and forth strokes to smooth and blend the surfaces. Along the edges, sand harder to feather the patch with the wall surface but avoid digging into the surrounding, sound drywall paper. Wipe the sanding dust away with a damp rag or sponge being careful not to wipe away the new joint compound.
Let the moisture from the rag dry and prime the patch. If you are using flat latex finish paint, it will be self-priming. If you are using semi-gloss or other shiny paint, prime with flat latex paint or a latex primer first before touching up the wall paint.