What's the difference between epoxy resin, polyester resin, and vinyl ester resin?

Epoxy resin is a two-component system with a liquid resin and a liquid hardener. When mixing epoxy resin, it’s very important to use the required mix ratio specific to each epoxy manufacturer, and to mix the resin and hardener thoroughly before adding any thickeners, fillers, or coloring agents.

When to Use Epoxy Resin: Because of its strength, epoxy resin is often used for wet layup in building or doing repairs on wood or fiberglass boats. Because of its durability, strength, flexibility and abrasion resistance, epoxy is preferred over polyester and vinyl ester in layups to sheath new wooden boats in a layer of resin and reinforcement material.

Polyester resin is typically used in fiberglass boat building. It will not cure completely in the presence of air, so wax is mixed in to achieve a proper cure. Polyester resin comes in two forms: a laminating resin that doesn't contain wax, and a finishing resin containing wax.

• 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 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.

To begin the curing reaction, polyester resin requires a catalyst called MEKP (methyl ethyl ketone peroxide). You can change the cure rate of polyester resin based on the percentage of MEKP catalyst you use, unlike epoxy resin, which requires exact amounts of resin and hardener to achieve a proper cure, 

The higher the percentage of MEKP, the faster the resin will cure. Common percentages are 1%, 1.25%, 1.5%, and 1.75%. In colder conditions, you can add up to 2% MEKP catalyst to speed up the 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 resin isn't commonly 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, iso-polyester resin is typically used in the external layers of a laminate, just beneath the gelcoat layer, and the less expensive ortho-polyester resin is used to add bulk in the laminate layers that are under the iso-polyester resin laminate layers.

Vinyl ester resin is chemically similar to polyester resin and both bond well to one another. As with polyester resin, vinyl ester resin also requires a catalyst, MEKP, to be mixed in to initiate the curing process; the resin and MEKP catalyst must be mixed thoroughly before adding any thickeners, fillers or coloring agents.

When to Use Vinyl Ester Resin: Because vinyl ester and polyester resins are chemically compatible, you can use them together in a layup to best advantage. For example, for blister prevention, you can use vinyl ester resin on the outer laminate layer (instead of iso-polyester resin), and use the less expensive polyester resin on the inner laminate layers to add bulk.