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.