Cracks in a plaster ceiling can spread quickly, covering a large area in a relatively short space of time. Although they look unsightly and can severely impinge on the aesthetics of a room, ceiling cracks can be repaired using techniques similar to any other plaster crack repairs. As a result, you can repair a crack in a plaster ceiling without having to expend additional energy to perform extensive redecoration. Once the cracks are repaired, paint the ceiling to look as good as new.
Scrape out any loose plaster in the crack, using a putty knife. Tap around the crack with the putty knife to loosen any other plaster. This will also show whether the plaster surround is properly attached to the lath beneath. If it is not, that section of plaster will need to be wholly replaced; otherwise, it will crack again. Brush away any loose plaster, dust and other debris, using a paintbrush.
Fill a spray bottle with water. Spray the plaster around the crack so it is moistened. Mix joint compound according to the manufacturer's instructions. Screw plaster washers into the plaster ceiling around the crack. The screws need to go into the lath in order to properly fix in position. Apply the joint compound to the crack and the surrounding plaster, using a trowel.
Wait at least 24 hours for the joint compound to set fully. Sand the surface level with the rest of the plaster ceiling. If the surface is not level, fill in any holes with more joint compound and allow 24 hours for it to set, then sand the surface again. Apply a coat of primer, and then paint the repaired area.
Before you start work on the ceiling, be sure to lay an old cloth or sheets of newspaper on the floor. This covering will protect the floor from any pieces of plaster that drop down, and also from dripping joint compound, primer or paint while you are making the repairs.
When it gets into your eyes, plaster can cause irritation and even damage. Always wear eye protection such as goggles or safety glasses when you repair a crack in a plaster ceiling; otherwise, pieces of plaster can easily fall into your eyes.