Hardcast Flex-Grip 550 Indoor/Outdoor Fiber Free White. 1 Gallon. Flex-Grip 550 is an all purpose, fiber-free, duct sealant for use on all types of metal duct, glass fiber duct board, and flex duct, as well as duct fabric and flexible tubing runouts. Distinguished by its ability to accommodate minor vibration and movement, Flex-Grip 550 stays flexible to save call-back labor. Flex-Grip 550’s excellent coverage rates and easy brush-on application provide low installed cost. • UL 181 B-M Listed • Passes ASTM C-731, ASTM D-2202 • USDA, EPA and FDA Approved • LEED compliant • SCAQMD Rule 1168 • City of Los Angeles Approval RR #8069