Polyurethane Sealer Reacts with Water

When freshly applied polyurethane sealer comes in contact with water,
the interaction produces a frothy white foam.

Question:
I applied a two-part solvent-based polyurethane sealer over stained concrete. In certain random areas, the sealer foamed and bubbled. What caused this foaming, and how do I fix it?

Answer:
Two-part polyurethanes are higher-end sealers for decorative concrete. Not only are they nearly twice as thick as an acrylic sealer (3 to 5 mils), but they also produce a very durable and chemical-resistant non-yellowing coating. The two components are made up of a resin (Part A) and a hardener (Part B). When these two parts are mixed at the proper ratio, they get very hard, with the time of hardening depending on temperature and other environmental factors.

One thing a lot of applicators aren’t aware of is that the polymer in the Part A resin reacts vigorously with water. The reaction creates bubbles in the sealer, which results in a frothy foam, as shown in the picture. (As a side note, this is how foam for mattresses, car seats, and foam insulation is made). In this case, water must have been present in this area when the sealer was applied.

The fix involves using a strong chemical stripper to remove the affected area. Remember that polyurethane sealers have good chemical resistance, so a typical solvent- or water-based stripper will have little or no affect. Once you strip away the foamed sealer, slightly sand the area and its borders to prepare for sealer reapplication. Then clean the entire area with alcohol or acetone to remove any residual stripper, dust, and dirt. Reapply the same sealer to the prepared area, taking care to blend the new sealer in at the edges. Finally, wax the entire floor to help blend the repaired area with the rest of the floor.