While WEST SYSTEM Epoxy creates an excellent moisture barrier by itself, 422 Barrier Coat Additive is a proprietary blend designed to further improve cured epoxy’s moisture exclusion effectiveness. 422 is used as a barrier coating additive to help prevent gelcoat blistering in polyester fiberglass boats. 422 also increases the epoxy’s abrasion resistance. Cures to a light gray color.
Add to mixed resin/hardener at the rate of 15 to 20% by volume or 3 tablespoons per 8 fl oz of epoxy. 16 oz can of 422 will modify approximately half a gallon of mixed epoxy.
Repairing individual gelcoat blisters from time to time and improving ventilation may head off the problem and avoid the need for a full-blown barrier job.