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WEST SYSTEM 422 Barrier Coat Additive is a proprietary blend designed to improve cured epoxy's moisture-exclusion effectiveness. 422 is used as a barrier coating additive to help prevent gelcoat blistering. 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 weight-3 tablespoons per 8 fl. oz. epoxy (approximately 32 oz. per B group).