Is Fiberglass Reinforced Plastic Waterproof? Is fiberglass reinforced plastic waterproof? In material terms, yes: fiberglass reinforced plastic (FRP) doesn’t rust and resists water when built correctly. In product terms, “waterproof” depends on edges, joints, fastener holes, and process quality. Use the right surface layer, closed‑mold processes, and proper sealing, and FRP panels and shells can stay leak‑tight in rain, washdowns, and outdoor exposure for years.