This story covers repairing loose plaster with joint compound.
Before you jump into a plaster repair, learn a little about how the plaster was applied when your house was built. That way, you'll understand how it came loose and what you need to accomplish to fix it.
When the plasterer pushed some of the first coat through the spaces between the lath boards, it oozed behind the strips, creating "keys" that lock the plaster to the strips. Impact or other damage can break the keys, and the plaster can sag from the ceiling or bulge on the wall. Then it's likely to crack.
In a repair job, your first task is to create a new mechanical bond between the surface plaster and the supporting framework. Using special plaster repair washers and screws, attach loose plaster to joists and studs where possible and to lath where necessary.
After you've secured the plaster, the repair proceeds essentially like a drywall finishing project: filling gaps with setting-type compound, embedding tape, and sanding to a smooth surface.
The project time depends upon the size of the repair; you also need to allow setting and drying time for the compounds
Stud finder, drill/driver, drywall mud pan and knife, utility knife
Using a stud finder, driving screws, applying tape and joint compound
Remove loose plaster chips, locate studs or joists
Plaster repair washers with screws, drywall tape, setting-type and premixed compounds, sanding screen
Remove any loose chips from the hole. Mark the location of joists or studs. Put plaster repair washers onto screws and drive the screws into framing lumber or lath surrounding the hole. Press the plaster firmly so the screw will pull it tight. To prevent cracking, drill pilot holes through the plaster using a carbide-tipped drill bit.
To speed the repair of large-scale holes, cut a piece of drywall that matches the thickness of the damaged plaster. Glue the patch to the lath and you'll minimize the number of fasteners needed, although you'll still need to drive screws into the framing members for extra security. You don't need to get a perfect fit around the edges because you can fill the voids with setting-type compound. To complete the repair, apply tape over the edges of the patch. Apply feathered layers of compound.