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.Liquid Hardener mixes with Bondo Extreme Filler or Bondo Liquid Resin for Fiberglass to create high-quality, long-lasting repairs. When hardener is mixed with Bondo Extreme or Bondo Liquid Resin for Fiberglass, a chemical reaction occurs to create a mixture that will deliver a strong, lasting repair. Our hardener is easy-to-mix and must be used with Bondo Extreme Filler - not designed to use alone.