Remove all the old, loose plaster. When working directly on a masonry surface, ... At the joint between the existing wall and ...

P laster sags from ceilings or bulges from walls when the plaster keys embedded around the wood lath break loose. To repair ...

Plaster-built walls usually don't have the same issue. If you are unsure about what type of wall you have, remove a wall plate around a light switch and look at the exposed section of the wall.