Plaster is a paste that is applied to walls and ceilings and dries to a hard finish. Although plaster is fairly durable, it is susceptible to cracking and developing holes over time. Luckily, repairing holes in plaster, such as in a ceiling, can be accomplished so that the hole is completely disguised. Repairing plaster holes in a ceiling is slightly more tedious than repairing wall holes because the hole is above your head and you must balance on a ladder.
Place a ladder under the hole and climb up. Enlarge the edges of the hole with a utility knife so that the hole is a square. Brush the surface of the hole with your hand to remove any loosened plaster that is still clinging to the ceiling.
Place a tape measure up to the hole and measure its length and width.
Transfer the hole measurements onto a 3/4-inch thick piece of sheetrock using the tape measure and a pencil. Cut along the pencil marks using the utility knife to cut out the square.
Spread 1 tsp of wood glue onto the back of the sheetrock in a thin layer. Press the sheetrock up into the ceiling hole and hold it in place for approximately 60 seconds. Place a piece of painter's tape over the sheetrock to hold it in place while the glue dries. Wait 10 to 15 minutes and then remove the tape gently.
Insert two to three drywall screws through the top of the sheetrock using a screwdriver to secure it to the ceiling underneath.
Apply strips of drywall tape over each of the four edges of the sheetrock. Press them down firmly with your fingers to remove any air pockets.
Open a container of ready-mix base coat plaster and scoop up 1 tbsp using a putty knife. Smooth the plaster over the sheetrock until it is completely covered. Scrape the putty knife blade over the surface of the ceiling to smooth the plaster out into the surrounding surface.
Apply a second thin coat to the patched area when the plaster is dry. Continue applying thin coats until the recessed area is no longer visible in the ceiling.