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The short answer is yes, a sagging plaster ceiling is dangerous. Sagging plaster on a wall is something you should fix immediately, but it isn’t a major concern. If the same type of sagging occurs on a ceiling, however, you could be in danger of your ceiling falling in large chunks. When plaster sags, it loses its strength and ability to hold itself up. The bonds that hold it together are strong, though, so if a sagging area falls, it will take large pieces down with it.

Depending on how much adhesive was used as well as the strength of the adhesive, you may have some drywall damage when removing a mirror. You can typically repair most drywall damage if it’s small by sanding the damaged area, applying spackle to smooth the damaged section so it’s flush with the rest of the wall, and repainting the area.

While it’s most common to sand plaster dry, it is possible to wet-sand plaster, too. Doing so can reduce the amount of plaster dust released into the air and make it easier to work in smaller, more delicate areas (such as around wall trim). However, some people find that wet sanding leads to more imperfections, which is why larger walls are more often dry sanded to ensure an even finish.

You can put certain types of plaster over other types, but doing so isn't always the best idea. For example, putting a coat of gypsum over lime plaster is possible, but doing so will limit the lime plaster's breathability and potentially affect its performance.

Since cement plaster tends to leave a textured surface, no matter how much you smooth it, some people prefer to use gypsum plaster as a topcoat over cement plaster, creating a smoother surface.