Mighty Tough Fibered Sealants are a UL Listed 181B-M, High Velocity, fast drying, non-flammable, high-quality, flexible water based duct sealant with excellent adhesion, specially formulated to meet SMACNA and local codes to pressure-seal all types of HVAC sheet metal air duct systems, and UL Listed flexible duct.Hercules Duct Sealer is made with the highest quality polymers that results in a consistant product that will perform in any HVAC duct applicaiton.