SG Red Bag. STOPGAP RED BAG is a self-smoothing underlayment suitable for use in light to medium-duty internal applications for preparing sound absorbent subfloors prior to the installation of new floor coverings. STOPGAP RED BAG is designed for application over concrete, sand/cement screeds and other solid absorbent surfaces. STOPGAP RED BAG is dimensionally stable and is supplied as a pre-blended dry powder which is designed for application between 3-6mm (unfilled). It is protein-free which allows it to be used in biologically sensitive areas.