Things You'll Need
Trowel (plaster knife)
Cleaning cloth or sponge
Plastic mixing tray
Ready-mix plaster of Paris
Be careful when using plaster of Paris to patch up ceilings because unlike setting-type compounds, it cannot be sanded upon drying.
Plaster of Paris needs a 24-hour drying time before it is paintable.
Drywall ceilings can become damaged due to moisture, improper curing, or poor workmanship. But patching small to moderate holes on ceilings with plaster of Paris is fairly manageable if you have good hand-eye coordination. Sometimes setting-type compounds are used to patch holes, but they do not bond as well to paper-faced plaster and do not achieve the more professional look of plaster. Before attempting to patch a larger hole, it may be a good idea to practice on a smaller one, especially for beginners, since plaster of Paris sets within 1 to 2 hours and leaves little room for mistakes.
How to Patch Ceilings With Plaster of Paris
Clean the surface of the ceiling. Scrape off any loose bits of plaster surrounding the hole either with your hands or the trowel. Then clean the area so it is free of dirt, grease, and grime using a cloth or sponge dampened with water. Also, dampen the edges and interior of the hole with water before applying the plaster.
Select the right-sized tool. A trowel is rectangular or triangular shaped and flat, which helps to apply the plaster evenly and efficiently. Trowels come in an array of sizes, and which one to use will depend on the size of the hole and what you can manipulate comfortably. If the hole is deeper, a larger trowel may be used, even if the diameter of the hole is small.
Mix the plaster of Paris. Fill one-third of a plastic mixing tray or container with cold water. Gently pour the plaster powder mix into the water. With your hands, gently shift the powder with the water to preventing lumping. Continue pouring the plaster into the water until the powder stands above the water level. Shift with your hands gently until the mixture has a smooth, milky consistency. The mixture takes several hours before it will harden. Do not use a metal container for mixing, because you will not be able to clean or remove the plaster after it hardens. You can find ready-mix plaster of Paris at any local hardware store.
Apply the plaster. Scoop a good amount of plaster onto the trowel and apply it over and around the hole. It's a good idea to start out with smaller holes around 2 inches in diameter to get a feel for the compound and the technique for smoothing out the surface. For holes that are deeper than 1/2-inch, fill them with plaster halfway.
Smooth out the plaster. Hold the edge of the trowel at an angle of 30 to 60 degrees, and scrape off the excess plaster, smoothing it out. Wipe the excess plaster off the trowel into the mixing tray, then scrape the plaster again with the trowel, repeating until the plaster is smoothed down to the level of the ceiling surface. Be careful when applying and smoothing out the plaster. If pressed too hard, the trowel can push the plaster too deeply into the hole, leaving an indentation that will look unsightly.
Apply another layer (if needed). Wait for the plaster to be almost dry before applying the second layer to fill in the rest of deep holes. Wet the trowel to get a sheen on the plaster. Repeat the previous step until the plaster is level with the ceiling surface.
Clean off. Use a sponge to wipe off the excess plaster around edges.
Nikki Cash graduated from UCLA with a bachelor's in film/TV, where she won an MPAA Scholarship. Since graduating, she has written and produced an independent feature film starting her career as a writer in 2005. Cash enjoys writing eHow articles with topics ranging from home improvement to crafts.