Repairing Cracks in Skim Coat

Bases

• The depth of a skim coat repair often depends on what underlies the material. Skim coats on wall board bond only to the wall board's paper covering. Cracks seldom require more than a smear of wallboard patching compound and sanding to smooth edges. If, however, a crack appears in the skim over lath and plaster, the repair may need to go into the underlying brown coat. Cracks in the skim might be hint at keying failure -- delaminating the plaster from supporting lath and requiring replastering.

Age

• Simple skim coat repairs require removal of loose plaster and recovering with patching compound. Owners of old homes, however, should ascertain the age of the plaster before choosing a patching compound. Before the 20th century, plasterers used a mixture of sand, water and lime, often with a handful or two of perlite or silica sand added to control shrinkage. The mixture for these old plaster walls is known as hydrated lime or lime putty, but old plaster recipes varied according to climate and to the skill of the plasterer. Later craftsmen used gypsum rather than lime because it did not shrink as much as lime putty. Wallboard patching plasters and plaster of Paris both use gypsum. Repairs must be made with a matching material so the cracks will not reappear.

Treatment

• Cracks must be checked for underlying problems such as loose keys, wall movement or wall board deterioration, and those problems should be addressed first. Clean and sand the area along the crack and dig out loose plaster with a utility knife or pointed end of a can opener until the plaster no longer crumbles and is solid. If possible, cut an inverted V so patching compound can be worked under the surface of adjoining plaster. Vacuum loose plaster and dust out of the crevice before proceeding. Wear gloves when handling caustic lime plaster and safety glasses to protect eyes against getting gritty airborne dust from any plaster.

Patching

• Spray the clean area with water to receive the patching compound and help it adhere to the old plaster. Hairline or small cracks need only a new skim with the patching mix, but larger cracks need two layers of skim -- one to fill the hole and one to complete the skim and level the patch. Drag the fill across the crack to push it under the surface. Sand the fill layer when it is completely dry, brush off the dust and re-spray the area with water. Drag the final skim layer along the crack to ensure a flat patch. Sand the skin when dry and prime the patch before painting.