Smooth and remove loose plaster pieces and any jagged edges on the surface around the crack with a putty knife. Use a cloth or rag to wipe away dust and debris. Dampen the area with a spray bottle and water. Follow the manufacturer’s instructions to mix dry plaster with water in a large bucket. Pour the plaster from the bucket into a mud pan. Apply a layer of plaster to the crack with a 4” wallboard knife, making sure the layer is even with the rest of the wall. Crack repairs usually need only one layer of plaster. Allow the area to dry for 24 hours.
When plastering, wear safety goggles, a long sleeve shirt and cotton work gloves for eye and skin protection. Wear a sanding respirator to prevent inhalation while breaking dried plaster or when mixing.
Excess moisture in the wall or room can affect the plastering, so make sure to fix any leaks or causes of moisture several weeks before plastering.
The best temperature for plastering is between 55 degrees and 70 degrees. Keep the room at this temperature at least 24 hours before plastering to ensure the walls are completely dry. The room must stay at this temperature during plastering and until the plaster has completely set.
You can also use spackle and a joint compound to patch cracks. For the smallest cracks, you can use spackle. Apply small amounts with a 2” putty knife until the crack is covered. Let the spackle dry for the amount of time specified by the manufacturer. Sand it smooth. For larger cracks, you can use a joint compound. First, apply a strip of self-adhesive fiberglass wallboard tape across the entire length of the crack. This will help keep the crack from growing. Apply the joint compound with a 4” wallboard knife over the tape until it is completely covered. Feather your coverage about 3” all around the taped area so that it will blend in with the rest of the wall. Allow it to dry and then sand it smooth with fine-grit sandpaper. Apply a second coat of joint compound and feather this layer out 6” to 12” from the repaired area to help it blend in further. Allow it to dry again before proceeding.
Wear safety goggles and a dust mask or respirator if you will be sanding a large repair area.