Epoxy resin provides better adhesion, moisture resistance, and flexibility compared to polyester resin. While polyester is more economical and widely used in mass production, epoxy is preferred for high-performance applications, repairs, and situations requiring a strong bond.

Polyester resin is widely used for building and repairing fiberglass boats. It is ideal for laminating fiberglass cloth, molding parts, and creating gelcoat finishes. It’s cost-effective and works well for general-purpose repairs.

Polyester resin cures in about 20–30 minutes at room temperature when mixed with MEKP (methyl ethyl ketone peroxide) catalyst. Cure time can vary with temperature and catalyst ratio.

Catalyze polyester and vinyl ester resin at about 1% to 2% MEKP by volume, with 1.25% to 1.5% a common starting point near 70°F, or roughly 1.5 to 2 ounces per gallon. Use less in heat and more in cold, and stay between about 1% and 3%. Too little leaves it soft and undercured; too much cracks the resin and cures it too fast to work.

Laminating (layup) resin stays slightly tacky as it cures so the next layer of glass bonds chemically without sanding. Finishing resin has a wax or surfacing agent that rises to the top, sealing out air so the last coat cures hard and sands clean. Most marine polyester is laminating resin, so add surfacing wax to the final coat when you are ready to sand and finish.

Yes, polyester resin can be tinted using compatible pigments. This is often done to match gelcoat colors or create custom finishes.

No, you should not use polyester resin on EPS foam (expanded polystyrene). Polyester resin contains styrene, a solvent that dissolves EPS foam on contact. This reaction will ruin your foam core and release harmful fumes. If you're working with EPS foam—for example, in surfboard shaping or foam core laminates—you should instead use epoxy resin, which is chemically compatible and safe for EPS.

Yes, resin has a shelf life. Unopened epoxy resin and hardener usually last one to several years stored cool and sealed, though hardener may darken without losing strength. Polyester resin and gelcoat are shorter lived, often six months to a year, because the styrene and promoters break down. MEKP also weakens over time, so old catalyst can leave resin undercured.