Bubbles in Coated Concrete Will Not Go Away

Air Moving Across the Epoxy

• Concrete is porous. Although it may appear smooth and dense, the aggregate in concrete actually contains tiny pores. Avoid allowing excessive air to travel across the surface of newly coated concrete. Air conditioning and windy days can introduce air into the epoxy, causing the formation of bubbles while the epoxy is in the process of curing. Once the epoxy is applied, there is no way for the air to escape.

Outdoor Temperature and Environment

• If concrete is coated in the morning, especially on a warm day, bubbles can form due to the expansion of heat in trapped air as the epoxy cures. Strong sunlight can also contribute to this problem. This condition is known as outgassing. Exterior concrete should be coated in the late afternoon after the outside temperature has peaked. As the outdoor temperature cools, it helps the epoxy to better adhere to the concrete and fill up the tiny voids.

Over-mixing the Epoxy

• Over-mixing the epoxy is another cause of bubbling. Over-mixing introduces air into the mixture, which can allow bubbles to form. Follow the epoxy manufacturer's instructions to the letter and use a slow and steady motion to mix the epoxy.

Surface Preparation

• In most applications, a vapor barrier should be installed under the concrete slab to prevent moisture in the soil from evaporating through the concrete surface. One solution for this problem is to coat the concrete with a high quality penetrating sealer and primer before applying the epoxy coating. If using a water-based epoxy, spray the concrete beforehand with water to fill in the concrete pores and push out the air then apply the epoxy to the wet concrete. Other solutions include applying a thin coat of epoxy, thinned with water, prior to applying a second and final coat of epoxy. One way to test for a recurring moisture problem is to tape a 2-foot square of plastic onto the concrete overnight prior to applying the epoxy. If moisture has formed under the plastic, this problem must be fixed prior to applying the epoxy. Bubbled epoxy will continue to recur if the moisture problem is not addressed.

Solutions

• The only way to remove bubbles in coated concrete is to sand them off. Sand the bubbled epoxy off with a rotary disc scrubber and medium-grit sandpaper or use an angle grinder. Abrade the concrete until all of the bubbles have been removed, vacuum off the concrete dust then re-coat the slab with epoxy.