Painting over plaster can enhance the look of the plaster, accenting the workmanship that went into the plaster application. Before painting though, the wall must be properly prepared, including repairing any damage. A flaw present in the plaster before painting will remain visible afterward. Fortunately, repairing the plaster is mostly a matter of creating a continuous surface. Most of the repair work only requires the application of a little filler, followed by blending in the patch to match the rest of the wall. It's simple and quick, and will have the wall ready to paint within days.
Widen hairline cracks in the plaster face slightly, using the edge of a putty knife. Remove a small amount of plaster from either side of the crack every 3 inches to create a gouge for the plaster to adhere. Remove any dust or loose plaster with a damp rag.
Scoop a small amount of patching plaster onto the end of a putty knife and push the plaster into the crack. Fill the crack completely, leaving a small amount of the plaster overflowing the area.
Go over the plaster with the flat of the putty knife, spreading it across the crack and about 1/4 inch to either side. Feather the edges of the plaster so that you blend the patch onto the surface of the wall next to the patched area. Remove any excess patching material with a damp sponge.
Fill larger cracks or holes in the plaster with patching plaster until you reach a point just below the rim of the opening. Allow all repairs made with the plaster 24 hours to dry.
Pack the rest of the crack or hole with joint compound to the same level as the plaster surface. Spread the compound just over the edge of the opening and level it with the flat of the putty knife.