A Foam Duct-Sealing System for 2-inch/50-mm or Larger Conduits. FST™ closed-cell duct sealant provides superior pressure-blocking power in the toughest environments. It stops methane and other gases to keep electrical systems intact. FST Duct Sealant is stronger, more chemically resistant, and cures 50 times faster than open-cell aerosol foams.