Q. We recently cleaned and resealed a decorative slab that was initially installed and sealed by another contractor. Everything looked good for the first months afterwards. Then, after rain showers, the sealer suddenly looked white and “frosty” everywhere. Are there issues of compatibility between the two sealers?

A. Sealers that turn whitish or scale after they are installed are issues that cause trouble for the entire decorative industry from time to time. It's further complicated by the fact that almost all decorative concrete work is sealed now—most of it with acrylic formulated sealers that provide a rich colored appearance. Not much has been written to date about compatibility issues between sealers, though there can be some. The problem you are experiencing may or may not be a compatibility issue.

One well-known compatibility issue has to do with putting sealers and coatings on top of siloxane and silane penetrating sealers. If you placed an acrylic sealer over a siloxane penetrating sealer, for instance, it would de-bond the acrylic sealer, causing serious scaling problems. This is made more difficult because it's impossible to know if a slab has ever been treated with these products by just examining a slab without testing it for the presence of penetrating sealers.

As a general rule, all acrylic-based sealers are compatible with one another, whether they are water- or solvent-based or sealers with different solvent products. But the solvent must completely leave the material before a product with another type of solvent is applied. And so, for example, solvent sealers may be placed over water-based sealers and vice versa, and a xylene solvent sealer may be placed over a mineral spirit solvent sealer.

Some less expensive acrylic sealers have lower molecular weight acrylic solids in them. These products are more susceptible to moisture issues and chemical attacks. Unfortunately, as a contractor, you won't know if you are using this type of product except by its performance over time.

Regardless of product incompatibility or product failures, the cause for the white appearance is moisture that has become trapped under the sealer.