Hardcast Duct-Seal 321 Indoor/Outdoor Fiber Reinforced Gray. 1 Gallon. Duct-Seal 321 in an all purpose industrial grade duct sealant for all types of metal duct, glass fiber duct board, and flex duct, as well as duct fabric and flexible tubing runouts. It includes UV inhibitors for extended outdoor exposure and built-in fiber reinforcement for added strength. This non-toxic water based product is solvent free and is suitable for residential use. -Duct-Seal 321 is UL 181 B-M Listed -Passes ASTM D-2202, ASTM C-731. USDA, EPA and FDA Approved. LEED compliant SCAQMD Rule 1168. -City of Los Angeles Approval RR #8069