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.
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