If your walls are slightly uneven or damaged, skim coating a light coat of drywall mud over the surface evens them out. Drywall mud is a thick substance that is spreadable and easily sanded to a smooth finish after drying. Choose between a dry powder you mix with water and a premixed mud. The drying time varies depending on the amount of mud you use and the humidity of the area, but the mud you mix yourself tends to dry faster.
Mix the drywall mud in a large bucket with an electric drill and mud paddle. If you purchased the powdered form, add water and stir. The premixed mud needs stirred also, and you may need to add water. Follow the directions on the package regarding the amount of water you need to add, as it varies with the manufacturer.
Continue to mix in small amounts of water to thin the mud down for rolling purposes. It is thin enough to use with a roller when it has reached the consistency of a milkshake.
Pour the thinned down mud into a paint tray. Use a roller with a 1/2-inch nap cover to roll three-foot wide by four-foot high sections, one section at a time so it does not dry before you have a chance to smooth it out.
Pull a drywall knife lightly over the wall. Leave just enough compound to evenly and lightly cover the wall. Sand with fine-grit sandpaper as needed.
Move on to the next section. Overlap slightly when moving to the next section to avoid heavy lines. Continue until you have finished the drywall that needs skim coated. Let dry thoroughly before applying paint.