- Architects and building owners select clear water repellents because they do not expect these coatings to alter masonry's natural appearance. Today, if a clear water repellent is carefully chosen, it is possible to ensure a clear finish. Pretesting on the actual masonry to be used helps in choosing the right coating. But, if specifiers and applicators don't take care, the following seven factors can discolor a wall. Ultraviolet exposure. Water repellents containing organic resins that are unstable under ultraviolet (UV) exposure will discolor over time. Most manufacturers now use UV-stable resins.
- Low water vapor transmission. The discoloration that usually occurs shortly after application is called "blushing."
- Oversaturated substrate. Blushing often occurs if the wall is oversaturated when the water repellent is applied.
- Chemical incompatibility with the surface. If the water repellent is not compatible with highly alkaline surfaces, the performance of some penetrating silicone-based coatings can be short-lived.
- Incompatibility with previous coatings. Sometimes, reapplying the same coating years later may cause clouding.
- Film formers. Unfortunately, when applied in sufficient quantity, most acrylics, stearates, and mineral gum waxes tend to darken the substrate's appearance or make it glossy. Silicones, too, can have a wetting or darkening effect, even a shine, depending on the angle of the light.
- Poorly formulated water-based products. Certain water-reduced materials formulated for use where volatile organic compounds are limited also may leave a blushed appearance.
The only way to ensure that a water repellent will not leave an undesirable appearance is to pretest it.