10 myths about epoxy: cured! Part 2


Last month, our epoxy expert David Johnson confirmed that epoxy definitely does stick to polyester – and it’s better value, too. In this, the second instalment in our myth-busting series, we tackle epoxy’s strength and find out just how good it is for repairs.

Myth 3: epoxy isn’t as good as polyester for repairs

It’s a strange myth, this one, because I’m not quite sure where it comes from. Yes, polyester resin is incredibly strong but when it comes to repairs, epoxy is so much better than any other resin system.

For one thing, epoxy repairs don’t shrink. Polyester shrinks because it contains styrene and those styrene molecules evaporate when the resin cures. On a small area that might be ok but when repairing a big area it will add stress to your repair before you’ve even got your boat back out on the water.

The other thing, of course, is inhaling all that styrene when mixing and working with polyester resin is not good – especially if you’re working in an enclosed space. It can make you feel light-headed, dizzy, tired… and that’s just for starters. Epoxy , on the other hand, contains no VOCs (volatile organic compounds) and is cleaner and easier to mix, especially if you use the simple pump set that you get with WEST SYSTEM® products.

There’s also a lot of evidence to suggest that epoxy is far more water-resistant than polyester resin, which is important for boat repairs that are below the water line.

It’s generally accepted that a polyester bond is about 20% weaker than a bond made with epoxy. However, if you want to get really scientific about water-resistance, many years ago our American cousins did an experiment on the “moisture exclusion effectiveness” of epoxy (and all sorts of other coatings) on wood.

Photo credit: Epoxyworks & Gougeon Brothers Inc.

It was in a lab and under very specific conditions but WEST SYSTEM 105 Epoxy Resin® with 205 Fast Hardener® came out on top, demonstrating more than 80% water resistance at 6 weeks of continued moisture exposure. After the same period of time, polyester resin was just over 30% water resistant. The speed with which the water resistance of polyester degraded also appeared to increase over time. (If you’re really interested, have a look at the graph in Fig 1, or read the whole report here.)

Myth 4: epoxy is more brittle than polyester

I’ve heard a few people ask if epoxy is brittle, especially when it’s used with wood. However, the chemistry of epoxy is such that it elongates to flex – even in a timber structure.

The typical elongation of polyester is between two and three percent, while with WEST SYSTEM epoxy it’s over four percent. This means that using epoxy will create a material that is far more resilient to constant loading and re-loading than polyester.

Take a lightweight dinghy, for instance; it will come under a lot of stress. It’ll be loaded time after time with dynamic loads from the water, the rig, the sails and so forth and if it’s built in polyester – which is not so good at elongating – it will microcrack. This results in a laminate that becomes softer and softer.

However, if you use epoxy it’ll be more resilient because it’ll stretch more. Post-curing the epoxy will also make it stronger and boost the elongation properties further still.

So, the simple answer to this myth is: no. It definitely isn’t!


Thanks very much to David Johnson for another expert contribution. Look out for part 3 next month!

Did you miss part 1 of our myth-busting series? Find it here.

Learn more about the full portfolio of WEST SYSTEM epoxy products.


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