Q.: We're applying a two-component polyurethane coating to the walls and floor of a new concrete reservoir. It's a high-build, 100% solids coating sprayed 125 to 140 mils thick in one pass. We first sandblast the concrete surface and repair any bugholes with a polymer-modified cementitious mortar. Then when the concrete is at least 30 days old we prime with a single-component aromatic urethane coating. We spray on the two-component urethane at a temperature between 140° F and 160° F. Our problem is pinholes, or what our workers call "glass eyes." These are voids right beneath the surface, covered by a thin translucent membrane of polyurethane that breaks easily. Is this caused by gases coming out of the concrete? How can we correct the problem?
A.: We asked Jim Kubanick of Coatings for Industry, Inc., to answer your question. He says that all urethane coatings cure by reacting with moisture in the air. The by-product of this reaction is carbon dioxide. When the urethane is applied at high temperatures, or at moderate temperatures and high relative humidity, bubbling may occur. Adding urethane-grade solvent to the polyurethane (up to 20% by volume) will help to prevent bubbling but it will make the urethane sag more when sprayed on vertical surfaces. You may have to apply the thinned material in two layers instead of one. Also, make sure that both the concrete and the bughole repair material have fully cured before you apply the urethane.