Sto Flexyl is an acrylic-based, fiber-reinforced, flexible adhesive and waterproofing for use on prepared concrete, masonry, plaster, gypsum and cement sheathing surfaces. Combined with Portland cement, Sto Flexyl provides multiple functions, including: waterproof adhesive and base coat in StoTherm^® ci wall systems, crack repair material as well as a foundation water-proofer.