Combine this additive with 105 Epoxy Resin to increase moisture resistance and protect fiberglass hulls against osmotic blistering.422 is a proprietary blend designed to improve epoxy's moisture-exclusion effectiveness. When blended into WEST SYSTEM Epoxy, Barrier Coat Additive helps prevent gelcoat blistering in polyester fiberglass boat hulls. It also increases epoxy's abrasion resistance. Light Gray.