Before painting plaster walls, you need to make repairs to cracked, loose or damaged plaster. If your plaster walls have imperfections, all of the time you take to paint will be wasted. You may need to replaster some areas, so plan a day or more for these repairs and for the plaster to dry. Small repairs may take only one day to dry, but large holes may take two to three days. Sanding will be the final process in preparing your plaster wall for painting.
Clear the walls of all pictures, nails and shelving. Look for flaws in the plaster wall. Shine a torch across the surface to illuminate imperfections and mark them with a pencil. Cover your floor and furnishings with a protective covering. Vacuum the walls to clear cobwebs and dust away and wipe down the walls with a damp sponge.
Scrape a plaster scraper over the damaged plaster to remove any loose particles. Use the sharp point of a bottle opener to scrape over small cracks so you can subsequently fill them.
Mix the patching plaster according to the manufacturer's directions. Apply three coats of patching plaster to the marks on the wall with a putty knife. Smooth the plaster over the marks in one direction and then more plaster in the opposite direction, and again at a 45-degree angle to fill all cracks and holes.
Apply one layer of plasterboard compound over the last layer of the patching plaster using the putty knife. Let the repair dry for 24 hours.
Sand the repair with a sanding block covered with a fine-grit sandpaper. Vacuum away the sanding dust. Wipe down the repair with a damp sponge to finish.