When a house settles, or shifts in its foundation, there are many signs. One sign is cracking in the plaster walls of the house. Smoothing out the plaster walls requires filling the crack and making the patch blend with the rest of the wall surface. Aesthetically, this can make a big difference in how your walls look and eliminate the impression that there are larger foundational issues with your home.
Wipe off the crack in the plaster with a damp sponge. This ensures that no dirt or debris inside the crack will prevent the plaster patch from adhering.
Mix the plastic patching powder with water according to the package instructions.
Fill the space with the plaster patch with a putty knife. Fill it until it is 1/8-inch from being even with the plaster wall. Wait 24 hours for the plaster patch to dry.
Apply joint compound to the plaster patch using your putty knife. Make sure that the joint compound is even with the wall, or only slightly higher than the actual plastic patch.
Allow several hours for the patch to dry. Sand down the joint compound using a fine grit sandpaper until it is completely even with the surrounding plaster wall. Paint and prime as needed.