BASF MasterSeal® 51720091 50 lbs 581 White Waterproof Cement-Based Coating - MasterSeal 581, formerly known as Thoroseal®, is a Portland cement-based coating for concrete and masonry that resists both air infiltration and positive and negative hydrostatic pressure. Polymer-modified with MasterEmaco A 660, MasterSeal 581 creates a low maintenance and highly durable waterproof barrier. It is waterproof to help protect building interiors from dampness and moisture damage.