Assess the surface of the wall or ceiling. Note whether it is smooth, rough or textured.
Attach a 60-grit sandpaper to a sanding block to sand rough or textured plaster surfaces. Sand the wall lightly with the sanding block.
Remove the 60-grit paper and attach 80-grit sandpaper. Continue sanding the wall until you are satisfied with the surface of the wall. Smooth the texture with the sandpaper until the wall can be painted with a brush or roller, depending on your preference.
Attach 120-grit sandpaper to the sanding block and sand a smooth-surfaced wall or ceiling until the surface is completely smooth and free of imperfections.