Bondo® Liquid Hardener mixes with Fiberglass Resin and Bondo® Metal Reinforced Filler to create high-quality, long-lasting repairs. When hardener is mixed with product, a chemical reaction occurs to create a mixture that will deliver a strong, lasting repair. Bondo® Fiberglass Resin Liquid Hardener is designed specifically for use with Bondo® Fiberglass Resin and Bondo® Metal Reinforced Filler. This hardener is easily mixed with resin or filler; a chemical reaction creates an extremely strong, waterproof repair material. The hardener must be used with a resin or filler for repairing autos, tubs, showers, sinks, pools, hot tubs, boats and more.