Old style plaster ceilings don't last forever, especially in humid areas like bathrooms. Small repairs can be easily done with drywall patching compound. Larger repairs may need to be patched in with sheetrock. It's not that difficult with some basic materials and tools and a professional looking repair can be done over a weekend.
Instructions
Repair Bathroom Plaster Ceilings
1. Assess the damage. Cover everything below the damaged area with drop cloths or plastic sheeting, get up on a ladder and tap the ceiling around the hole or crack with a putty knife. It will sound hollow wherever the plaster is starting to come away from the lathe. Plaster walls and ceilings are constructed differently to modern sheetrock. Thin strips of lathe are laid across the joists. A coating of mortar, often mixed with horsehair (or even feathers) is spread over the lathe, and allowed to dry. Once dry, a thinner coat of white plaster is applied which gives plaster walls their smooth and seamless appearance. Since homes with plaster walls or ceilings are usually quite old, age eventually takes its toll on the plaster.
2. Repair cracks without removing much of the plaster if the surrounding area doesn't sound hollow and feels solid. Using the edge of the putty knife, widen the crack by scraping out loose material.
3. Apply a coat of drywall mud followed by a strip of fiberglass mesh tape, pressed smoothly into the fresh mud. Allow this to dry for a minimum of 8 hours. The fiberglass tape will be more flexible than regular paper tape, and less likely to crack later. It is also somewhat easier to work with.
4. Using a 10 inch drywall knife, apply another smooth coat or mud over the repair. The wider knife allows for sanding later to feather in the edges of the fresh mud with the surrounding surface. Let this dry completely, and sand it with a sanding block or 120 grit sandpaper wrapped around a piece of two by four. You want a flat sanding surface to sand the repair flat and smooth. Prime this with PVA drywall primer and repaint the ceiling.
Repair a Large Hole in Plaster Bathroom Ceiling
5. Repair large holes by patching them in with sheetrock, or with a metal patch. Remove as much loose plaster as you can. You will get down to the lathe, and it's impossible to simply apply drywall mud over it as a patch. If the hole is less than six inches square, buy an adhesive metal mesh patch at any hardware store. Stick it to the ceiling and apply drywall mud over it - usually it will take two coats to look good - and sand flush.
6. Use sheetrock to fill in larger damaged areas. Using a hammer and a rigid putty knife, remove the old loose plaster until you have a square or rectangular area to patch in. Cut a piece of sheetrock to fit, and screw or nail it to the joists. You may have to remove enough plaster to reveal a joist so the sheetrock is nailed at each edge.
7. Apply drywall mud to the seams, followed by a length of drywall tape, smoothed and embedded into the fresh mud. As with the step above for filling cracks, let it dry and reapply mud in a smooth, wide application. If you are either very good, or not very particular, two coats of mud might suffice, but very often a third coat is necessary to fill in air bubbles, ridges and low spots.
8. Sand smooth and prime the area with PVA drywall primer. If you are particular, you will easily be able to see any irregularities you missed earlier, once it's primed. You can touch up rough spots, re-sand and re-prime. Now you are ready to paint.
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