FlexSeal? Caulk Grade Sealant is a white solvent-based synthetic elastomeric sealant. FlexSeal is extremely flexible and durable, but with a higher viscosity formulation than Standard FlexSeal. Like all solvent-based products, the surface must be clean, completely free of moisture and residual contaminants before application. FlexSeal? CaulkGrade Sealant is especially suited for use in any application where caulk is typically used.