A skim coat added to a plaster wall removes the bumps and depressions in an old plaster wall's texture or hides a new plaster wall's imperfections. The smooth, flat surface of skimmed plaster readily accepts wallpaper or new texture. The base coat a plaster installer applies to a wire-lathe-covered wall consists of at least 2 layers of a gypsum-based plaster. Once the base coat dries, the installer works a skim coat into the plaster wall's base coat. Remodelling contractors usually do not remove an old plaster wall's texture before applying the skim coat.
Dampen a sponge in water and clean the plaster walls.
Mix a bucket of all-purpose joint compound with an electric drill equipped with a cage bit until the joint compound's consistency matches warm butter. Premixed joint compound comes from the manufacturer in a bucket with all the ingredients included. Professional plaster installers commonly mix the plaster manufacturer's recommended ratio of fast-set powdered joint compound, lime and water in a 5-gallon bucket.
Skim a coat of the joint compound 1/16 inch thick onto the plaster wall with a 12-inch flat trowel. Start at the top of the wall and feather the skim coat toward the floor. Cover the entire wall.
Hold a 24-inch flat trowel at a 45-degree angle and use the edge of the trowel to smooth the skim coat. Start at the top of the wall in 1 corner and work toward the floor. If the skim coat starts to stick to the trowel's blade, wet the blade with water.
Let the skim coat dry completely, using the joint-compound manufacturer's dry-time guide. Premixed all-purpose joint compound takes 24 hours to dry. Fast-set joint compound states the dry time on its bag, usually between 5 and 30 minutes.
Set a light on the floor next to 1 corner of the plaster wall. Shine the light across the wall's surface. The skim coat's imperfections and ridges will show up as shadows.
Dampen a sanding block in water.
Remove the ridges and imperfections in the plaster wall's skim coat with the damp sanding block, using the shadows from the light as a guide.
Apply a second skim coat if the first skim coat did not hide all of the plaster wall's imperfections or texture.
Work on 1 wall at a time. Amateurs should avoid using fast-set plaster with lime due to its quick-setting nature.