Epoxy Sealer in Sawcuts Leaves White Haze

By Chris Sullivan – ChemSystems, Inc.

Question:

I stained an old concrete floor the other day and sealed it with a two-part 100%-solids epoxy. I love the way the epoxy looks, but there are minor imperfections, such as trapped lint and small bubbles, that keep the surface from looking like a perfect sheet of glass. Also, there are really deep sawcut lines in the floor (1 to 2 inches), and the epoxy that went into the sawcuts stayed a milky white color.

What is the best method to apply the epoxy to avoid these issues?

Answer:

Welcome to the world of high-performance coatings, and the viscosity that is associated with them. Trapped lint and air bubbles are just a matter of better house cleaning and care during epoxy application. Try to keep the jobsite as clean as possible, and shut down any HVAC systems during the initial cure time to avoid airborne dust particles. When applying the epoxy, wear medical shoe covers, such as those used in hospitals, rather than cotton socks, which give off lots of lint. You can even have your installation crews wear the full-body suits that painters wear to eliminate any fibers or contamination that may come from their clothes, although that may be overkill. Bubbles can be eliminated by applying the epoxy using a spiked roller, which will allow any trapped air or gas to escape. Midwest Rake is a good source for this type of roller. (See this page from their catalog.)

As for the white haze that developed in the sawcuts, the cause is either trapped moisture or the coating was applied too thickly. The fact that the sealer is a 100%-solids product and does not need evaporation to cure leads me to believe that moisture is the culprit. Cracks and joints are common places for water to hide. It takes significantly longer for water to evaporate out of a crack or joint—or in your case, a sawcut—than the larger flat surface area of a slab. With such deep sawcuts, you need to give the floor plenty of extra dry time or use a leaf blower or high-pressure air hose to speed drying. Unfortunately, the chance of the white haze going away on its own with a 100%-solids product 1 to 2 inches thick is slim. You may need to grind out or chemically strip the coating from the sawcuts. When you go to reseal the cuts, make sure they are dry and fill them with multiple thin applications of epoxy vs. one thick application. Thin coats of materials will give any trapped air or gas a better chance to migrate to the surface and escape.

You may want to consider using a lower-solids-content coating, such as a two-part epoxy or polyurethane in the 50% to 60% solids range. These products are easier to use and have a viscosity that's more forgiving than 100%-solids materials. Plus, they will provide almost the same performance when applied in multiple coats.

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