Resins

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.

Choose vinyl ester when you need more strength, toughness and water and chemical resistance than polyester without moving to epoxy. It is the standard for blister-resistant hull skins, below-waterline repairs and tanks. It cures with MEKP like polyester, about 1.25% at 77°F, and goes over polyester or vinyl ester laminate, so it drops straight into a polyester layup schedule. It does not go over epoxy.

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.

It depends on the fabric, the area and the number of plies. For hand layup, plan about 0.73 oz of resin per square foot per ply of 6 oz cloth, 1.22 oz for 10 oz cloth, 3.4 oz for 1.5 oz chopped strand mat and 4 oz for 1708 biaxial, then add 15 to 20% for waste. Mat takes about 2.3 times its own weight in resin. The resin calculator works out a full layup.

The standard dose is 1.25% MEKP by volume at 77°F, about 12 cc per quart or 47 cc (1.6 fl oz) per gallon. Use more when it is cold and less in heat, but stay between 1% and 2%: below 1% the resin can stay soft, and above 2% it can crack, smoke or kick before you can work it. Measure in cc with a dispenser, not drops.

Hardener speed sets working time and cure time, not final strength. West System 205 Fast gives 9 to 12 minutes of pot life (100 g at 72°F), cures down to 40°F and suits cool shops and small batches. 206 Slow gives 20 to 25 minutes for warm shops and bigger layups, and 209 Extra Slow about 40 to 50 minutes. Always mix at the ratio the system specifies.

Yes. 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 three months to a year depending on the product, because the styrene and promoters break down. MEKP also weakens with age, and old catalyst can leave resin undercured, so check the label.