COVERAGE A 20kg bag of STOPGAP 700 SUPERFLEX mixed with 5.0 litres of clean water will cover approximately 5.0m2 at a thickness of 3mm. STOPGAP 700 SUPERFLEX is a fast drying, fibre reinforced self-levelling smoothing underlayment designed for use on plywood, sand/cement concrete and steel subfloors prior to the installation of new floorcoverings.