Q: I am having trouble sealing my exposed-aggregate driveway. I'm told to do this once a year but the sealer won't hold up that long. The first time I used a water-based sealer it turned a milky color after 3 months. I tried that product again with the same result. Next, I tried a brown tint acrylic sealer that did the same thing after 6 months. Across the street, a neighbor washed his driveway one day and sealed it the next. I had been pressure washing in the morning and sealing in the late afternoon. This time I tried his way. With a sudden, unexpected rain the next day, I now have a few milky spots and tire tracks. My question is, can I apply another coat on top? Or do I have to pressure wash and start over? Am I doing or not doing something to cause the milky, white spots?

A: This is not a simple problem, and water-based sealers complicate the issue further because moisture and temperature conditions are more critical than with solvent-based systems.

Often, whitish areas on sealers indicate that the sealer has lost its bond to the concrete and light reflecting through that small air space causes the white appearance. If that is the case, all the white marks must be removed completely prior to resealing. Another possibility with water-based products is that if the coalescing agent in the sealer does not evaporate in the proper sequence in relation to the water in the sealer, the acrylic molecules will not join together in the proper manner, and that will create the white appearance. In this case too, the white material must be removed completely because it will reflect through a new coat of sealer.

You might start with a pressure washer. You can go as high as 3000 psi and bear down on the surface to remove anything that is not bonded well. Pay close attention to the concrete to be sure you aren't removing any part of the surface. Then a stripping agent should be used to remove any white-colored areas that remain. A contractor recently told me that he used a product called Citrus Strip by Dayton Superior that worked well for him (after trying two similar products that didn't). He used a buffing machine with a strip pad to work the stripper into the sealer. You also can use solvent strippers, but if you do, find one that has a surfactant included, which will allow the dissolved sealer to mix with water for easy cleanup.

In general, we don't recommend water-based sealers outdoors and prefer to use solvent-based acrylics. There are fewer problems with them, and if there are problems, they are easier to repair. The best seem to be 100% acrylic sealers-ones that do not have other polymer additives. A major concern is the "breathability" of a sealer outdoors. Acrylic resins transmit higher amounts of water vapor than other polymers.