Painting a concrete block wall serves two basic purposes. The first is aesthetic. Painting a concrete block wall a colour other than its natural grey helps the wall better match the decor of its surroundings. The second reason is maintenance. The painted surface is easier to keep clean than the unpainted blocks. The key to success when painting a block wall is surface preparation. Once the blocks are properly prepared, the remainder of the job is mostly about covering them with smooth, even coats of primer and paint.
Remove loose or peeling paint from the blocks. Scrape the loose paint away with a putty knife and then scrub the area with a wire brush.
Clean the surface of the block wall. Use plain water to remove dirt and debris. Use an acidic masonry cleaner to remove any white powder on the concrete blocks from efflorescence caused by salts and free alkalis leeching from the block interior. Scrub the powder away with a scrub brush and rinse thoroughly to remove the acidic residue. Clean any stains resulting from oil or grease deposits using a solution consisting of 28.4gr of trisodium phosphate mixed with 1 cup of water and 1 cup of baby powder mixed to a paste. Spread the paste on the stain, allow it to dry and then scrape the paste and oil away.
Measure the surface area of the wall to determine the amount of paint needed. Start by measuring the length and height of the wall, then multiply the measurements to get the square footage. Purchase 1 gallon of paint for every 200 square feet of wall space. If painting split-face concrete blocks, the kind with the rough surface rather than the smooth, then purchase 1 gallon of paint for each 100 square feet of wall space. Purchase the paint that you're applying to the walls as well, using the wall measurement to determine the amount of paint. The amount will vary according to the paint manufacturer. Purchase acrylic or elastomeric paint for a long-lasting surface in the colour of your choice.
Apply a layer of primer to unpainted blocks to fill in pores and provide an even-coloured surface for the base coat of paint. Pour the primer into a painter's tray and use a paint roller to apply the paint to the block surface. The paint roller should have a ¾-inch nap for smooth block coverage and 1 1/4-inch nap for split-block usage. Apply the primer to the blocks with smooth even passes. Roll the primer on firmly, pressing it deeply into the pores of the blocks. Work in vertical lines when you apply the primer, taking care to cover the surface completely. Allow the primer to dry for the time suggested by the paint manufacturer.
Caulk any expansion joints and around any windows or doors located on the wall. Measure the length and width of the expansion joint and then cut a tube of foam backer rod to fit. Use backer rod slightly thicker than the expansion joint so that it can expand and contract with temperature changes. Stuff the backer rod into the expansion joint, leaving a space of between 1/2 to 3/4 inch from the top of the rod and the surface of the block wall. Fill the space with polyurethane sealant. Level the sealant on the surface of the wall using a plastic spoon dipped in white spirit to smooth the sealant. Apply elastomeric caulking around the edges of windows and doors. Place a bead of caulking along the seam between the door or window and the blocks. Smooth the surface of the caulking with a wet finger. Allow the caulking to dry for two hours before continuing.
Apply the final coat of elastomeric or acrylic paint using the same process as the primer application. Cover the wall completely, rolling on an even coat of the paint. Allow the paint to dry and examine the coverage. If you can see through the primer, or the coating is uneven, apply a second coat of paint and allow it to dry completely.
Always wear rubber boots, rubber gloves and safety glasses when working with the acidic cleaner to prevent burns. Paint in a well-ventilated area.