The Ratio of Hardener to Polyester Resin

by Cassandra Tribe
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Polyester resin is recommended for use with fiberglass repairs that are above the water line on boats and also on other dry-surface fiberglass forms such as auto bodies and models. When preparing to perform a repair, you must mix the proper ratio of hardener to polyester resin to prevent the resin from failing to cure properly.


The hardener used with polyester resin is also called a catalyst. The catalyst agent creates a chemical reaction that allows the resin to change from a liquid to a solid state. This transformation is referred to as "curing" or "polymerization." Methyl Ethyl Ketone Peroxide (MEKP) is the chemical hardener used with polyester resin.


Catalyst agents are usually sold with the hardener but can be purchased separately as well. The agent is packaged in a small bottle or vial that is marked to show how much of the agent has been dispensed from the package in drops or fractional teaspoons. The correct ratio of hardener to resin should be calculated before the mixing process is begun to make it easier to monitor how much catalyst to dispense.


The ratio range for catalyst to resin is 1 to 2 percent hardener to the total volume of resin to be used. For example, four drops of hardener will be 1 percent of 1 ounce of resin. Adding more of less of the catalyst agent will speed up or slow down the curing time for the resin. However, adding less than 1 percent or more than 2 percent will cause the resin to cure improperly and the repair to fail.

Special Considerations

Several factors should be taken into consideration when you're estimating the amount of catalyst to add to polyester resin to achieve the curing speed desired. The thickness of the mat or laminate, the temperature and humidity or other weather conditions can all effect curing times. Only mix enough resin and hardener as can be applied within a half hour of combining the two, as the resin will begin to set or become hard after that time.

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