QUESTION: We have a retaining wall built of interlocked decorative tan blocks, and they have been "leaking" a chalklike stain. Do you have any recommendations for removing the stains and/or preventing them?ANSWER: Most likely, the staining is caused by efflorescence, a common and stubborn problem in brick and block masonry. According to the Masonry Institute of America, moisture gets into the wall and brings sodium within the stone to the surface. This results in crystals of salt covering the wall and creating a white, chalky substance."A lot of times there's really nothing you can do, because masonry surfaces are porous," said Dan Willard of Holmlund Masonry Restorations in Minneapolis. "These surfaces do breathe, they do soak up moisture and they also allow moisture to exit."Although nothing has been proven 100 percent effective against the problem, Willard offers these tips to remove and prevent the stain.• Remove the deposits with an efflorescence cleaner, available at home-improvement and masonry-supply stores.• Prevent the stain from returning by applying a breathable masonry sealer.• Limit the amount of moisture the surface absorbs by placing a waterproof barrier between the wall and whatever it's retaining.For more information, visit the Masonry Institute of America: www.masonryinstitute.org.