Polyester resin is commonly used in building fiberglass boats. It does not cure completely in the presence of air, so wax is typically mixed in to achieve a proper cure. For this reason, polyester resin comes in two forms, a laminating resin that does not contain wax, and a finishing resin that contains wax.

• Polyester finishing resin – This resin contains wax, which allows the resin to cure. Finishing resin is typically used as the final coat in a layup or laminating schedule.
• Polyester laminating resin – This resin remains tacky to the touch and does not cure in the presence of air. This makes it ideal for layups because you want the build up laminate layers to achieve a proper bond.

Polyester resin typically contains an accelerator (styrene or cobalt) to ensure that the resin can cure at room temperature. The type of acid used in polyester resin is either orthophthalic (ortho-polyester) or isophthalic (iso-polyester). Orthophthalic polyester resin is the cheaper, more brittle of the two. Isophthalic polyester resin is also more chemical resistant and water resistant.

Styrene can also be used to thin polyester gelcoat resin when spraying. To initiate the curing reaction, polyester resin requires a catalyst called MEKP (methyl ethyl ketone peroxide). You mix in the catalyst when you’re ready to use the resin.

Unlike epoxy resin, which requires exact amounts of resin and hardener to achieve a proper cure, you can manipulate the cure rate of polyester resin based on the percentage of catalyst mixed into the resin.

The higher the percentage of catalyst, the faster the resin will cure. Typical percentages are 1%, 1.25%, 1.5%, and 1.75%. In colder temperatures, you can sometimes add up to 2% catalyst to expedite a cure.

It’s important that you heed the resin manufacturer’s minimum and maximum catalyst percentages and don’t over-catalyze or under-catalyze or the resin won’t cure properly.

When mixing polyester resin, it’s also important to mix the resin and catalyst thoroughly before adding any thickening agents or fillers.

When to Use Polyester Resins 

Because of its brittleness, ortho-polyester is not typically used for structural layups. Iso-polyester resin is not as brittle, and is slightly more water resistant and chemical resistant than ortho-polyester resin.

For this reason, it is typically used in the external layers of a laminate, just beneath the gelcoat, and the less expensive ortho-polyester resin is used in the laminate layers to add bulk beneath the iso-polyester resin laminate layers.

Advantages

Polyester resins are relatively inexpensive and are compatible with gelcoat finishes. Polyester resins are compatible with CSM (fiberglass chopped strand mat), so it dissolves the binder in the CSM and cures to a strong composite. CSM is typically the last laminate layer beneath the gelcoat paint due to its strength and smoothness.

Disadvantages

Polyester resin is weaker and more brittle than vinyl ester resin, and has a relatively short shelf life (1 year) compared to epoxy resin (3 years). Shelf-life should be monitored carefully, because this resin will eventually harden in the container.

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