Plaster is a hard, durable finish for walls. For many years it was the traditional surface for all interior walls, until it was largely replaced after World War II by plasterboard or drywall. Even plaster can deteriorate over time and can develop cracks and rough spots. Removing wallpaper from plaster can leave gouges and holes that must be smoothed before new wallpaper or paint is applied. The solution is called skim coating, applying a thin layer of material to cover any blemishes and leave a clean, smooth surface.
Prepare the wall. Remove any nails, screws, picture hangers or other objects. Fill those holes and any large cracks or gaps with joint compound and a drywall knife. Repair cracks by scraping out a slightly larger area with the knife, to be sure to get rid of any loose pieces, then fill with joint compound. Use a fast-drying compound, and sand the repairs smooth as soon as it dries. Repeat the patching if necessary to get an even surface.
Sand the wall with a sander and fine sandpaper to get it as smooth as possible. Check for any remaining holes or blemishes that might not have been filled on the first check, and fill and sand those. Wipe the wall with a damp sponge to get rid of any dust, remaining wallpaper paste or other debris. Paint the wall with primer, or add a special plaster bonder to the compound if there is any doubt about adhesion of the skim coat.
Use joint compound or a special skim coat plaster; compound works well in most situations and once dried and finished will be almost indistinguishable from plaster. Get two varieties, a fast-drying type for a first coat, and an all-purpose compound for a second and third finish coat if needed. Cover baseboards, door and window trim, electrical boxes and any other components on the wall with blue painter's tape.
Start at the top of the wall and work down, using a very wide drywall knife, 8 or 10 inches wide, and a plasterer's trowel or a tiling trowel, which has one serrated side and a second straight flat side with a big handle. Whichever tool you choose needs to cover a wide area. Put compound in a pan or similar container. Pick up a big glob with the trowel and spread it quickly over the surface as smoothly as possible. Cover a large area of the wall, then go back and smooth out any ridges or bumps. Keep working until the whole wall is covered, then move to the next wall.
Let the first coat dry, then scrape off any ridges or bumps with a knife or a utility rasp. Get that first coat smooth -- sand if necessary -- then add a second, thinner, coat to cover any imperfections in the first coat. Use a slightly thinner compound mixture if the original is not flowing on smoothly; thin compound with water. Cover the wall with the second coat and let dry. Sand it smooth with fine sandpaper, and apply a third coat if needed to make the wall perfectly smooth.
If you're a beginner at skim coating, try a test on a small section, like under a window or behind where a couch is normally placed, until you get the hang of it. You can wipe off any unsatisfactory test with a wet sponge before the compound sets.