Chimney Rx Masonry Crack Joint Sealant is a brushable elastomeric sealant used to repair exterior cracks in mortar joints on chimneys, brick walls and between concrete or stone masonry units. The product seals larger cracks and other defects that cannot be corrected with water repellents alone. Chimney Rx Masonry Crack Joint Sealant seals cracks up to 1/8 inch wide on mortar and brick joint interfaces to provide waterproof protection. It dries clear to milky white, depending on the size of the crack. Product prevents water infiltration and subsequent damage, yet allows water vapor to escape.