RCD Corporation® #8 Mastic® is a water-borne sheet metal air duct sealant used for permanently sealing the joints and seams of all duct types including UL-181 listed air ducts. RCD Corporation® #8 Mastic® is the best choice for high velocity sheet metal air duct construction used in auditoriums, high rises, hospitals, industrial ventilation systems, large retail stores, malls, military buildings, prisons, schools, sports centers and complexes. It remains flexible and withstands the constant joint movement of these demanding systems.