WATER REPELLENTS ARE TRANSPARENT COATINGS formulated for the purpose of protecting porous substrates by preventing the penetration of liquid water. Unlike waterproofing materials and sealers, water repellents allow the passage of water vapor and generally are not designed to prevent the intrusion of liquid water under hydrostatic pressure. Water is known to penetrate porous materials and contribute to their deterioration [1–3]. Examples include warping and swelling of wood as well as cracking and spalling of concrete and masonry caused by freeze/thaw cycles and dissolved salts. Further, chloride ions in de-icing salts can accelerate the corrosion of reinforcing steel in concrete. Water repellents are used to provide protection against such damage. Water repellents provide protection by depositing hydrophobic compounds on the substrate, thus modifying the surface tension of the treated area. With some treatments, water may bead on the treated surface.