How to Seal Block Walls Before Painting

Wash the concrete wall with a pressure washer set to 2000 psi. This step will remove any dirt, grease or dust, which will enable the sealer to stick better to the concrete block. Use plain water for new or relatively clean walls, but add detergent to the pressure washer for walls that appear greasy or contain efflorescence deposits. Scrape off any hard-to-remove particles with a hand scraper or scrub brush.

Inspect the mortar joints and repair any loose or damaged mortar. Premixed acrylic mortar is available and it dries quickly. Follow the manufacturer's instructions and patch or replace the mortar as needed.

Prime the block wall, using special acrylic block fill primer and applying it with a paint roller. This layer will seal the pores in the concrete blocks, helping the final coat of paint to adhere better and resist peeling. Apply primer to the mortar joints, too.

Seal all cracks in the concrete wall with elastomeric caulk, following the manufacturer's directions. Use foam backer rod for the expansion joints, placing them behind the caulk, according to the manufacturer's label directions.

Apply one to two top coats of the desired paint color. Use an electric paint sprayer and then quickly roll over the surface with a paint roller. The best type of paint for exterior concrete walls is elastomeric. For interior walls, use a gloss or semi-gloss for the easiest maintenance.