Repairing small cracks in plaster is easy compared to plastering a whole room. To repair small cracks in plaster, you’ll need patching compound, medium- and fine-grit sandpaper, a sanding block, a putty knife, a masonry chisel, a paintbrush, a rag, and a screwdriver. For finishing, you'll need primer and paint.
Remember: Plaster repair is much more difficult than fixing drywall. Unless the repair is small, don’t attempt any plasterwork unless you’re trained or experienced. It’s easier to replace a wall or ceiling panel with drywall.
Use the chisel to widen and undercut the crack.
By making the back of the crack wider than the front surface (undercutting), you create an opening that the plaster can grip.
Scrape out all loose plaster with the tip of the putty knife, and wipe away the dust with the rag.
You don’t want any loose particles ending up in the plaster patch.
Thoroughly dampen the full length of the crack with a wet paintbrush.
Dampen inside the crack, as well. The damp surface will help the plaster adhere well.
With the putty knife, apply a thin coat of plaster patching compound over the crack.
Be sure to feather the edges. Plaster patching compounds can vary, so follow the manufacturer’s directions for use.
Let the patch dry completely (usually overnight), and smooth the patch with sandpaper wrapped around a sanding block.
Begin with medium-grit (80-grit) sandpaper to smooth out the bigger bumps. Then, switch to a fine (120-grit) sandpaper to get a fine, smooth finish.
Paint the patch with primer.
Don’t skip this step or you’ll end up with a permanently goofy-looking, non-matching area.
Apply touch up paint to the patch in your wall color.
This is why you saved that half a gallon of leftover wall paint.
