Question: I have a water leakage problem in the brick and block walls of a 20-year-old, 4-story building. I applied a clear coating to the building and tested it after several days with a Rilem tube but found no improvement. Why? Is the coating inappropriate or did I test too soon?

Answer: You can test a siloxane penetrating sealer or a film-forming sealer the next day, says Pat Gorman of Gorman Moisture Protection, El Paso, Tex., president-elect of the Sealant, Waterproofing & Restoration Institute (SWRI). But you should wait at least 5 days and preferably a month before testing a silane penetrating sealer. And be sure you measure the wall's water permeance before applying the sealer, so you will know the sealer's true performance.However, I don't recommend using Rilem tube tests to measure the moisture penetrating the surface of a wall. It tests less than 1 square inch of a masonry joint, and unless it's placed in precisely the same spot both before and after applying the sealer, any comparison of results is meaningless. A field version of the ASTM E 514 water penetration test, though more expensive than the Rilem test, measure water penetration through a 12-square-foot area, and its results are thus more meaningful.You also may have used the wrong sealer for your particular substrate. One way to determine the best sealer for your substrate is to apply several sealers and test them first.

Be careful in choosing a sealer. Some are not only ineffective in reducing water penetration; some may also cause damage to the substrate.The lack of improvement may have nothing to do with the sealer or the test method you used. Both film-forming and penetrating sealers repel water, but don't expect them to seal large separations or cracks. If large separations or cracks exist in the mortar joints, you must repoint the mortar joints or repair the cracks in the affected area before applying the sealer. Clear coatings can correct some masonry problems, but they're often not cures by themselves. To stop leakage, you usually also must repoint joints, repair flashing, or take other corrective measures.For other information about sealers, consult Clear Water Repellents for Above-grade Masonry, available from the Sealant, Waterprooofing & Restoration Institute, 3101 Broadway, Ste. 585, Kansas City, MO 64111.