ThermX Basecoat Sealant is a brush grade caulking material that stays flexible and fills gaps and holes prior to elastomeric or paint coating on roofs or walls. It's lightweight and very stretchy while being extremely waterproof. Use if to fill joints in fascia, siding, on metal roof joints, on stucco or wood to seal out wind and rain or on tile roofs as the basecoat part of a waterproofing system.