Before beginning repairs to walls or ceilings that have been damaged by water, locate and stop the source of the leak or you will be repairing the same damage over and over again. Also be sure the plaster has time to dry out completely before attempting to start repairs.
About Plaster Water Damage
When three-coat plaster gets wet the surface will usually show some visual signs. A brown stain may appear in ceilings or walls, or with a heavy soaking, the surface may begin to bubble from the chemical reaction of the lime. As the surface dries, it may harden and the surrounding paint will flack off. Water damage effects may be minor, isolated to the surface layer of the wall, or it may penetrate the whole 3-coat structure, causing crumbling brown and scratch coats that fall off the lath. In some cases, the lath may also be damaged, showing rotted wood or rusted metal under the plaster coats.
Plaster that has gotten wet is best repaired using joint compound, will not be hard enough and is vulnerable to any residual moisture in the wall material. Repair deep damage to the sand coats or lath, use the process at this link. Repair surface damage, use the process below.
#1- Remove the Damaged Plaster
The first step to making repairs is to remove all the damaged material. This includes all the bubbling and peeling paint, crumbling plaster coats, and any degrading lath.
Use a putty knife to lift off flaking surfaces. If the damage goes deeper, apply firm pressure with the blade to dig out all the soft material. Go around the edges of the damaged area, digging with the knife until you reach hard, undamaged plaster.
Test the paint coat immediately surrounding the area to see if it will peel off easily. Work out from the center with a putty knife to remove any loose paint.
#2- Remove the Dust and Prime
Go over the surface with a stiff nylon brush and shopvac to remove any loose particles. Wash the surrounding surfaces using a sponge and bucket of clean water to remove any residual lime deposits and let the wall dry completely before proceeding.
With the surface clean and dry, coat the area with oil-based or . Priming is essential to block any residual contamination that may remain in the surface, inhibiting the bond with the patching compound. It will also prevent stains from bleeding through the finish paint which could result in bubbling of the new paint coat.
Stir the primer, or shake the can vigorously to be sure it's completely mixed. Coat all affected surfaces and the surrounding wall paint. Let it dry completely before beginning repairs.
Quick-dry primers will dry in about an hour, oil paint may take as long as 24 hours to dry completely.
#3- Repairing the Plaster
Repair shallow surface damage using several thin coats of joint compound to fill in the missing plaster, and level out the wall. and cover the whole damaged area and surrounding wall with a ¼inch coat.
Float the blade across the deeper damage to fill it in and skim harder on the surrounding walls to leave a very thin coat. Don't let mud build up higher over the repair area than it is on the surrounding surfaces.
If you have a large area to repair, use a long, thin board like a piece of lattice molding to scrape off the high points. Ignore any ridges left by the knife blade or other inconsistencies in the surface, when the mud has set you can scrape them off before applying another coat.