Our experts answer contractors’ biggest questions on coating concrete.

Preventing sealer blush

Q. How can I prevent a sealer from blushing?
A. The best way to prevent blushing is to make sure the substrate is clean and dry. Blushing usually occurs from contaminants, moisture or solvent entrapment, excessive film build, and temperature and humidity during and after application.

People sometimes refer to “white stains” as blushing, but they’re actually contaminants or efflorescence. Like blushing, these can also develop on the concrete surface under the sealer. The best way to prevent them is to prepare the surface properly before sealing. Most often, white stains occur because traces of an earlier coating, moisture or contaminants remain on the concrete when a new sealer is applied. Pressure-washing usually isn’t enough to thoroughly remove previous coatings or contaminants and adds unneeded moisture to the concrete. It’s better to use mechanical means, such as grinding, mechanical scarifying or blasting, to make sure the concrete surface is completely clean and dry before you apply a new sealer.

It’s important to note that white stains that appear on the surface of sealed concrete are sometimes just a sign that the sealer is working as intended. In other words, it’s keeping water from being absorbed by the concrete. When water evaporates, it can leave behind lime or calcium deposits. It’s worth trying to remove any such stains before concluding that the sealer has failed.

Excessive film build is self-explanatory. The best rule of thumb is to follow the manufacturer’s application directions on labels or product data pages. Most data pages include surface preparation, application, cautions and temperature limitations.

Beware of "cure and seal” products on stamped concrete

Q. After cleaning and pressure-washing, I applied a clear sealer on a client’s stamped concrete driveway, and after six months it failed. What could have caused the failure and how could I have prevented it?
A. Some contractors who place and stamp concrete driveways often specify concrete with integral color, pour it and stamp it. Then, to save money, they come back three days later and apply a styrene-based cure-and-seal product. The problem is, the cure-and-seal is incompatible with all acrylic sealers and will prevent them from adhering properly to the concrete.

If you’re asked to seal stamped concrete flatwork, test it first for absorption. A dime-sized bead of water dropped on the surface should be absorbed within five or 10 seconds. If not, that’s a sign the concrete has been treated with a cure-and-seal product and shouldn’t be recoated. You’ll need to strip the concrete bare first. When it’s dry, use any H&C^® Decorative Concrete Sealer, based on the amount of sheen you want.

Avoiding bubbles and blisters in coating

Q. What causes a protective coating on an exterior wall to form bubbles and blisters soon after it’s applied?
A. Bubbles and blisters can be caused by several things: contaminants left on the surface, existing coatings, and solvent or moisture entrapment. Often contractors will pressure-wash a building as a way to prepare the surface for painting. This helps remove dirt, chalk and loose flakes of old coatings. But cold-water power-washing alone may not always remove grease, oil and other contaminants. If there are oily contaminants present, a good emulsifying agent/cleaner/degreaser should be used in conjunction with the power washing, giving the new paint film a better chance to adhere to the substrate. Existing coatings may also be a contributing factor to blistering and peeling. This is generally seen when the existing coating isn’t compatible with the new coating or has a very tight film. Again, preparation is critical.

Finally, moisture entrapment. Moisture can come from many sources: roof leaks, downspouts, preparation of the substrate and interior air-conditioned spaces. Leaks need to be addressed prior to surface preparation. Moisture entrapment can also result when a surface is power-washed and painting begins before the substrate is dry. This issue is most often seen in masonry. Painting the same day as power-washing generally is not a good idea.

Before you apply an architectural protective coating, you want the surface to be clean, sound, dull and dry. You can’t necessarily tell if the wall is dry enough just by looking at the outside of it. It’s important to be aware of what’s going on inside the building as well. If you’re preparing to coat the exterior of a concrete block building, for example, remember that the interior may be air-conditioned at 70°F. Even if the outside temperature is 95°F and the wall feels warm and dry, there may be moisture condensing inside the block that can cause blistering as it tries to migrate out. To be on the safe side, use a moisture meter to make certain the exterior surface is dry enough before you paint.

Why concrete coatings peel

Q. What might make an exterior protective concrete coating peel after just six months?
A. One of the most common reasons a protective coating peels is due to existing unstable coatings left on the surface when the new coatings are applied. Even though the substrate looks sound and stable, the existing coatings’ adhesion may not be. Generally, it’s one of the underlying layers that fails, not the new coating. The new coating’s adhesion is only as good as its foundation, and when it’s suspect, it’s best to remove the old coatings. It’s always important to prepare the surface properly if you want a protective coating to perform successfully.