Applying plaster to a concrete wall can change its appearance, level it, make it more moisture-resistant and give you the opportunity to add texture. Plastering a concrete wall takes time. You will have to prepare the wall and allow for curing and drying time between applications of plaster. A poorly-prepared surface will eventually crack and sag and the plaster will come off the wall. Old paint, dirt, mould and mildew all create problems.
Prepare the concrete wall. If the wall has been painted and the paint is peeling or flaking, all of the paint has to be removed. The easiest way to do this is to have the wall sandblasted.
Install metal lath over the painted concrete wall if you don't want to sandblast. Use 7/8-inch concrete screws with washers spaced 12 inches apart to attach the lath to the wall.
Caulk any cracks and treat mould and mildew by first killing the mould and then removing any stains left by the mould. Clean mortar joints with a wire brush.
Use a bonding agent on painted walls, when the paint is securely bonded to the wall. Spray or brush the agent over the entire wall. Follow the manufacturer's instructions.
Apply a dash, slush or slurry coat to the concrete wall. Mix 1 part Portland cement, 1 to 2 parts of masonry sand and enough water to create a runny consistency. Use a dash brush to cover the entire wall to a thickness of about 1/8 inch. Let the slush coat dry for a day. Dash, slush and slurry all refer to the technique of creating a base coat. For a very flat finish, use a slurry that is almost liquid in consistency.
Mix plaster according to the manufacturer's instructions. Apply a base coat over the dash coat that's between 1/4- and 3/8-inch thick with a trowel. Smooth the plaster out for a flat wall. Leave the wall rough for a textured look. Let the base coat dry.
Put a second coat of plaster over the base coat using the same technique and plaster mix.
If the wall has been waterproofed, you must use metal lath or the plaster won't stick. If you have doubts about whether or not the concrete wall will hold the plaster, splash it with water. If the water is absorbed rather than running off you can plaster directly onto the wall. Plaster cures best at 12.7 to 21.1 degrees C.
The finished depth of the plaster should be between 1/2 inch and 5/8 inch.