Cracks or holes in plaster walls can be repaired with a little patience and a bag of patching plaster. Premixed joint compound or spackling will not adhere to plaster, and though there are other products on the market that may work, in the end, plaster to plaster is your best bet.
Step 1: Prepare the area to be patched by removing all loose plaster and debris. Undercut the edges of the crack or hole so that the patching plaster can seep under the edge and lock itself into place.
Step 2: Following the directions on the package, mix the plaster up to a cake frosting-like consistency, the less water the better. For large areas you may want to add a bonding agent in lieu of water.
Step 3: Use a spray bottle or sponge to dampen the area to be patched and then use a drywall or putty knife to fill the area with wet plaster. If the hole is more than a half-inch deep, fill it only half-full, score the plaster with the knife edge and allow it to dry before proceeding.
Step 4: Choose a flexible drywall or putty knife that is approximately 2 inches wider than the hole. Dip the knife in water and smooth the wet plaster so it is level with the surrounding surface. Holding the blade at a 30-degree angle while it spans the hole will make the job easier. A light touch is imperative, as pressing too hard will create a depression in the center of the patch.
Step 5: Use a damp sponge to wipe any excess plaster off the surrounding area. Rinse the remaining plaster from your tools and mixing container. Plaster will harden within two hours, so time is of the essence.
Step 6: Once the new plaster has begun to set, use a damp sponge to gently wipe the patch to a smooth finish. Allow the plaster to cure completely before painting over it. Original lime plasters used to take over a year to set, but today’s gypsum base plasters usually set within a few days. I suggest waiting at least a week to be safe. Remember, slow and steady wins the race, and patience is a virtue we should all embrace.