Polywater FST-250 Foam Duct Sealant is a specification grade, two-part, high-expansion foam duct sealant. It keeps water, acids, dust, gases, insects, and rodents out of the duct. FST™ Sealant expands and hardens to a “closed cell”, rigid structure. It is both permanent and removable. The foam expands and cures quickly, even when water is present.