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- 1). Clean out all of the old, damaged plaster with a small, flat pry bar, or a flat-head screwdriver, by gently dislodging it from the ceiling. Remove any broken wooden laths using the small, flat pry bar and a claw hammer. Insert one end of the pry bar underneath the laths and gently tap with the hammer until the lath comes loose. Remove any remaining nails with the claw of the hammer. Use a small brush to sweep out any remaining dust and debris.
- 2). Replace any broken or rotted wooden lath. The strips can be various sizes, but they need to be similar to the ones already in place. Purchase treated lath material from your local hardware store that is 1 1/2 inches wide, 1/2 inch thick and 8 feet long. Cut the lath to the desired length and reattach with 1-inch drywall screws. Leave about 1/4 inch between the lath so that the mud can be pressed in-between. Pre-drill the holes into the lath--it will split if you fail to do so. Because ceiling rafters are spaced 24 inches apart, it may be necessary to remove the plaster all the way back to 24 inches in order to replace the lath.
- 3). Mix the 90-minute drywall mud with water in a 5-gallon bucket, using a mixer that attaches to a drill. Add about 2 qt. of water first. Don't mix too much mud at once; it will set up in 90 minutes, so work quickly. Use 45-minute mud if you want to work faster. Pour about half the amount of powder as water in the bucket and mix. Add more powder so that you have a mixture that is consistent with toothpaste.
- 4). Work the drywall mud onto the lath and press it into the gaps between the lath. Use the 4-inch putty knife that you applied the mud with to cut lines into the mud, horizontally and vertically. Create a surface that the next layer can adhere to. Repeat the steps until the mud reaches the original surface, and you can feather the mud out onto the plaster about 6 inches all the way around.
- 5). Sand the drywall smooth once it has dried. Allow the mud to dry overnight before you attempt to sand it. Although it will set in 90 minutes, it won't be fully ready for sanding. Once the area is sanded smooth, prime and repaint it to match the original surface.
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