Whitening of sealer is often caused by applying it to a wet surface.

Moisture is a leading cause of problems with decorative concrete sealers. Under certain conditions, moisture can become trapped in or under the sealer, resulting in whitening or clouding of the sealer membrane. But why does this happen, how do we avoid it and how do we fix it?

There are two key contributors to moisture problems. The first is sealer contact with moisture in the concrete during application. Cures, cure and seals, and sealers for decorative concrete (see Part 1 for the differences between these product categories) are all designed to handle different levels of moisture contact. Cures and cure and seals can handle higher levels of moisture contact, allowing them to be applied to green (high-moisture-content) concrete and not turn white or cloud up. Decorative concrete sealers, on the other hand, can't take much moisture contact. This is why they need to be applied after the concrete has cured for 28 days. If a decorative sealer is applied to green, or wet, concrete, you can pretty much guarantee the development of a nasty white haze. This has to do with the type of resin (or plastic) the coating is made from, and how that resin deals with moisture contact.

The second key contributor to moisture problems is the permeability of the sealer, or how readily water is able to pass through the sealer membrane. Permeability is directly related to the solids type and content and thickness of the sealer. All exterior acrylic cures, cure and seals, and sealers are designed to allow some level of permeability when applied at 300 to 500 square feet per gallon. The lower the solids content and/or the thinner the membrane thickness, the more moisture that can pass through the sealer without getting trapped and turning white. This is why applying sealer at the proper thickness is so important, especially when dealing with high-solids-content products (in excess of 25%). The higher the solids content, the smaller the margin of error. Most of the moisture-related problems I see in the field are caused by overapplication of high-solids cure and seals or sealers.

In regard to avoiding moisture-related problems, it is really quite simple. Use a sealer with a solids content of less than 25% and apply it thinly by spray. If problems do occur, misting solvents over the surface, such as acetone, xylene or MEK (methyl ethyl ketone) followed by back rolling will spread out the sealer film and remove excess material. After the solvents evaporate, the sealer will reharden. In a worst-case scenario, it may be necessary to strip off the sealer followed by cleaning of the surface and sealer reapplication.

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Author Chris Sullivan, ConcreteNetwork.com technical expert and vice president of sales and marketing for ChemSystems Inc.

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