Guide to Polyester Resin Marine Applications

Polyester resin has been used to build and repair boats since fiberglass boats were first invented. In fact, polyester resin is a popular marine resin of choice thanks to its ease of use, versatility and affordability. 

There isn’t much polyester resin can’t be used for in regards to building or repairing boats. Although epoxy and vinylester resins may provide stronger bonding, or better water and corrosion resistance, polyester is still the most commonly used resin throughout the marine industry.

Polyester Resin Repairs

Polyester resin is excellent at wetting out and laying up fiberglass, which makes it perfect for making repairs on fiberglass boats. It can be used to laminate chopped mat or fiberglass fabric to plug holes, fill gouges or cracks, and create a solid, uniform surface. 

Once cured, polyester resin provides the best resistance to UV light of all marine resins, which is precisely what you want if the area being repaired is outside of the boat. Polyester resin can also be sanded and finished with gelcoat, and if done right the repair should be flawless.

NOTE: Gelcoat can’t be applied to epoxy resin, so polyester resin is the better choice for external and top layer repairs. Polyester resin doesn’t bond well to vinylester and epoxy resins. 

Reinforcing with Polyester Resin

Polyester resin can also be used to wet out and lay-up fiberglass to add additional layers of lamination around areas that are subjected to stress, such as stringers or bulkheads. This method of reinforcement has long been a popular use of polyester resin.

Working with Polyester Resin

Polyester resin is great for fabricating parts and making molds of almost any shape or size thanks to its ease of use and effectiveness at fiberglass wet out and layup. 

It has a high viscosity and thixotropic index, which means it tends not to run or drip once applied, even on vertical surfaces. 

NOTE: Polyester resin has a relatively short shelf life of about 6-12 months from the date of manufacture. As such, it’s generally better to purchase it only when needed instead of in bulk.

Mixing Polyester Resin

Polyester resin must be mixed with a catalyst, which is usually methyl ethyl ketone peroxide (MEKP), to begin the curing process.

The amount of MEKP used determines the speed of the curing process, with more for a quicker cure, less for a slower cure. However, if a fast curing time is required, the MEKP “dilutes” the resin and reduces its properties (strength, resistance, etc.), so the less MEKP used the better. Once polyester resin is mixed with the catalyst, the curing process can’t be stopped. The resin must be used before the curing process completes, and any mixed but unused resin should be discarded.

NOTE: All industrial chemicals are hazardous, and the fumes given off by MEKP are particularly toxic and flammable. Care must always be taken when using polyester resin. 

Why Polyester Resin

When it comes to building or repairing boats, polyester resin has been used for a wide array of applications. Epoxy and vinylester resins may have some advantages over it, but polyester’s ease of use and affordabilty means it’s still the most popular and widely used marine resin out there.