Hard water stains on brick walls can be as tenacious as an uninvited fly at a picnic. Generally, they’re composed of calcium carbonate and you’ll need an acid-based cleaner to get rid of them. However, don’t use muriatic acid (which is commonly suggested), because it can stain or bleach many colors of brick and cause aluminum window frames to corrode. Instead, several brick cleaners are available that, because of special buffers, work well without the staining problem. Your local brick dealer probably carries these products. But if not, ProSoCo and Diedrich Technologies offer reputable exterior brick cleaner products so you can learn how to clean exterior brick.
When you clean the brick, remember that you are using an acid-based product and follow all precautions and directions. After cleaning your brick, avoid some future headaches by sealing it with a siloxane or silane-based sealer (available from the same sources listed above). To find out more information on cleaning, visit the Brick Industry Association.