HT Flexseal Silicone. HT Flexseal 300 is an elastomeric Silicone Sealant for use where good adhesion and a flexible bond is required, with high temperature resistance. This is ideal for gasketing, sealing and bonding of materials to withstand temperatures of 300°C. Having an excellent adhesion between most substrates, suited for gap-filling applications and jointing materials of different thermal expansion & where vibration is a problem. BOX OF TWELVE. . Please ensure you check that your area is included in our free delivery policy and exclusions and if so please email us for a delivery price to your area.