Q. My company has done plaster repair work for many years. After repairing water-damaged walls or ceilings, we occasionally get called back to "fix" an unsuccessful repair. Our second repair attempt involves digging out a very bumpy, chalky substance, and more often than not, we have to repeat this process a number of times until the patch finally takes hold. What causes this reaction on some water damage jobs and not others? What is the most efficient way to deal with this problem?
A. Mel Hines responds: The problem you refer to is caused by efflorescence — salts in the plaster are brought to the surface by the intruding water. The water often causes the magnesium in the lime coat to expand and produce the blisterlike effect you refer to.
Assuming that the water intrusion has been stopped, the first step is to chip away the lime coat at the affected area. There is a good chance that the bond between the finish lime coat and the plaster base coat has been weakened.
When faced with this situation, I use a wire brush to scrub away any loose base-coat particles, then apply a coat of Kilz (Masterchem Industries, 3400 W Segerstrom Ave., Santa Ana, CA 92704; 866-977-3711) to the base-coat plaster. Kilz is an alkyd-based sealer, primer, and stain blocker. The plaster base coat must be completely dry before application. Next, I apply a coat of Durabond (United States Gypsum Corp., 550 W Adams St, Chicago, IL 60661; 800-950-3839). Durabond is a fast-setting, low-shrinkage compound with tenacious bonding qualities and accelerated setting times.
Finally, I apply a skim coat of ready-mixed all-purpose joint compound. After a light sanding, the repair is ready for painting.