Can TotalBoat Polyester Fairing Compound be used below the waterline if top coated with gelcoat?

TotalBoat Polyester Fairing Compound should only be used below the waterline if a barrier coat product is used over it, and it is not being used as a structural filler. Gelcoat applied as a finish material for Polyester Fairing Compound should not be considered impervious to water, but can be used for cosmetic purposes.