Coremat is a thin polyester bulking mat filled with microspheres that adds thickness and stiffness to a fiberglass laminate while using less resin than extra glass. It also controls print-through so the weave does not telegraph to the surface. Lay it between glass layers, wet it out fully with polyester, vinyl ester, or epoxy resin, and it conforms to curves. Use 2mm for light bulking and 4mm where you need maximum stiffness.