To fix a crack, run an old screwdriver or a beverage-can opener along it to enlarge the opening and knock out loose particles. If a crack is over 1/4 inch wide, undercut its edges: that is, taper them inward to widen the crack at the bottom. Brush out loose plaster. Fill the crack with spackling compound: smooth with a putty knife.
To fix a hole, check to see if the sup-porting wood or metal lath is sound. If it is, clean out loose particles and undercut the edges. Wet the edges thoroughly with water and apply a plaster-bonding agent. Then, starting at the edges and working in, fill the hole to about half its depth with patching plaster. Roughen the plaster’s surface with the edge of a putty knife and let it dry for 24 hours. Then dampen the area and apply a second layer of plaster, smoothing it even with the wall.
If the lath is broken, use a keyhole saw or metal sheers to remove dam-aged portions. Clean and undercut the hole’s edges. Cut a piece of wire mesh larger than the hole, and loop a length of wire through it’s center. Put the mesh in the hole and pull it tightly against the inside of the wall. Twist the wire around a pencil or dowel to hold the mesh in place. Dampen the hole and apply plaster-bonding agent. Spread patching plaster generously around the edges; make sure it penetrates the mesh. Let it dry. Then re-move the wire and finish plastering.
Finish all patches by sanding with fine paper. Prime before painting.