When moisture gets into masonry walls it can cause many problems. Applying a clear water repellent, though, can reduce the amount of water penetration. Some water repellents are even guaranteed to work for 10 years. But with or without a guarantee, a water repellent only works if it's the right product for the job and applied correctly. COMPARATIVE SHOPPINGThe Sealant, Waterproofing & Restoration Institute (SWRI) has drafted a list of proprietary water repellent products that includes more than 75 acrylics, 20 silicones, 30 silanes, and 50 siloxanes. Compare the four main types of water repellents: acrylics, silicones, silanes, and siloxanes. Silicones and acrylics form a surface film that is highly water repellent. No chemical reaction occurs; the film is simply left behind after the solvent evaporates. Silanes and siloxanes, on the other hand, penetrate the masonry. They form a water repellent barrier by undergoing a chemical reaction. After reviewing the major types of water repellents, check the manufacturers' literature for the properties of specific products. SWRI recommends checking for: water repellency, water vapor transmission, gloss, weathering and ultraviolet stability, resistance to efflorescence, and water permeance.PROPER APPLICATION IS A MUSTWater repellents only work if they're properly applied. The masonry surface must be clean and dry. Applying sealers, especially acrylics and silicones, to a wet wall can trap crystal deposits behind the coating and cause spalling. Also, as a rule of thumb, only apply water repellents when the air temperature is between 50 and 90 degrees Fahrenheit.