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,
Inc., P.O. Box 368, Barnhart, MO 63012;
314/942-2510) 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., 125 S. Franklin,
Chicago, IL 60606; 800/552-9785). Durabond is a
fast-setting, low-shrinkage compound with tenacious
bonding qualities and accelerated setting
Finally, I apply a skim coat of ready-mixed
all-purpose joint compound. After a light sanding,
the repair is ready for painting.
Mel Hines owns Atlanta/Pro-Serve, a ceiling
and wall repair service in Atlanta, Ga.