“White scum,” or a more common condition known as efflorescence, is responsible for the whiteish staining seen on brick walls.
White scum is a condition that surfaces if brick is cleaned too early after construction, cleaned with the wrong material or just cleaned incorrectly. A way to verify this condition is to wet the brick. While the brick is still wet, if you can still see the white on the brick then this is very likely what you have.
Ordinary brick cleaners will not break down the white. You will need a special brick cleaner such as SureKlean White Scum Remover marketed by ProSoCo, a leading manufacturer of masonry cleaning and restoration products. The White Scum Remover should be diluted at a rate of one part cleaner to one part water. Let the solution dwell on the brick for three minutes and rinse. A pressure rinse of 1,000 pounds will help, if it will not harm your brick. Some soft and more porous brick should not be pressure-washed, as it will damage the face of the brick.
Efflorescence is another condition that leaves a white film on the surface of masonry. Trace minerals found in some clay and cement products cause it. Basically, it is a water-soluble salt, which forms when moisture has been drawn out of the brick and dried on the surface leaving a residue of salts.
Muriatic acid is not a recommended treatment, although some masonry suppliers in the past have specified it. Usually, the staining caused by efflorescence is easily removed when the wall has dried completely. A product such as a light-duty concrete cleaner, available from you masonry dealer, should be sufficient.
Usually, this condition will not return unless your brick is subject to a lot of moisture. If the problem persists after the initial cleaning use a masonry sealer after a thorough cleaning.
Another recommended cleaner is ProSoCo Weather Seal Siloxane PD or, for extremely porous surfaces, use ProSoCo Custom Masonry Sealer. For information on this product, contact ProSoCo at www.prosoco.com, or call 800-255-4255.
Cleaning products vary, so be sure and follow manufacturer’s directions carefully for any product you selection.