Nelson Firestop Putty (FSP™) is a completely premixed and ready-to-use formula that can be hand pressed into place forming an immediate fire seal. When exposed to fire the intumescent material expands to seal voids created from fire exposure on deteriorating penetrating items. FSP™ is a blend of organic and inorganic materials that is hand moldable. FSP™ is reusable when adding or changing penetrating items since the material remains pliable and does not set up.