Trade Secrets: 10 Myths About Epoxy: Cured!

Have you ever considered using epoxy resin for a project but been hesitant due to circulating rumours? In the first instalment of our epoxy myths busting series, our in-house technical expert, David Johnson, debunks common misconceptions about this versatile material.

Myth 1: Epoxy Doesn’t Stick to Polyester

Experienced boatyards and yacht surveyors widely understand that epoxy is the most reliable method for conducting durable, long-lasting glass fibre repairs on polyester or vinylester hulls, despite the frequent misconception.

In fact, a properly executed epoxy repair, finished with a polyester gel coat, often outperforms traditional polyester repairs in terms of both strength and longevity. The key factors for success are ensuring complete epoxy curing, thorough abrasion, and thorough cleaning.

Polyester gelcoat is generally not recommended for finishing repairs below the waterline.

For underwater areas, applying three to four coats of epoxy before using a bottom paint is the preferred approach. This is because cured epoxy maintains its dimensional stability and avoids shrinkage, unlike polyester repairs which can become thick, bulky, and develop visible edges.

To guarantee sufficient curing, boatyards typically use heat lamps. However, for DIY projects, allowing a full seven-day curing period is essential to prevent the epoxy from reacting when the polyester gel coat is applied. Additionally, thorough abrasion of the epoxy repair before applying the top coat is crucial.

Myth 2: Epoxy Is More Expensive Than Polyester

Yes, that’s correct. Epoxy costs more per kilo than polyester. However, the fact is, you won’t use anywhere near as much epoxy as you would polyester resin. You’ll also use less fibre and other materials too.

Within the scheme of an epoxy composite boat build, for example, you’ll need far less epoxy to build a strong hull than you would if you were building the same size boat in polyester. This is because cured epoxy is much stronger than polyester resin. You can afford to use less fibre with it too because the sheer strength of the epoxy will make the fibre you do use much stiffer.

For example, a 12mm boat hull in polyester resin could easily be reduced to 4mm in epoxy and would be incredibly strong but also quite flexible. In fact you could reduce the skin even more to 2mm and put a foam core in – laminating over the back of the foam to create a sandwich – for a structure that’s just as strong but even lighter. Essentially, it’s possible to engineer an epoxy boat to be both strong and light whereas with polyester you’d have to choose between one or the other.

So, despite the initial higher cost, epoxy can be a more economical choice for boatbuilding projects, especially those prioritising speed and efficiency.

Myth 3: Epoxy Isn’t As Good As Polyester For Repairs

It’s a bit of a head-scratcher, this one. I mean, where did this myth even come from? Polyester resin is definitely strong, but when it comes to repairs, epoxy is just on another level.

For starters, epoxy repairs don’t shrink. Polyester, on the other hand, does.

This is because it contains styrene, and when the resin cures, those styrene molecules evaporate. On a small area, maybe it’s not a big deal. But if you’re repairing a larger area, the shrinking can actually add stress to your repair before you even get your boat back in the water.

And let’s not forget about the health hazards. Working with polyester resin can expose you to harmful styrene fumes. This can make you feel dizzy, lightheaded, and tired, and that’s just the beginning. Epoxy, on the other hand, is much safer. It doesn’t contain any volatile organic compounds (VOCs), so it’s cleaner and easier to work with, especially if you use a handy pump set like the one that comes with WEST SYSTEM® products.

There’s also plenty of evidence to suggest that epoxy is far more water-resistant than polyester. This is crucial for boat repairs that are below the waterline.

While polyester bonds are about 20% weaker than epoxy bonds, water resistance is another story. Years ago, some American scientists did a study on the “moisture exclusion effectiveness” of different coatings, including epoxy. The results were pretty clear: epoxy was a clear winner when it came to keeping water out.

So, there you have it. Epoxy is the superior choice for boat repairs. It’s stronger, safer, and more water-resistant. So, next time you’re faced with a repair, don’t be fooled by myths. Go with epoxy and you won’t be disappointed.

*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.

Myth 4: Epoxy Is More Brittle Than Polyester

I’ve heard this one a few times, especially when people are talking about using epoxy with wood. But the truth is, epoxy is actually quite flexible. Unlike polyester, which tends to be more brittle, epoxy can stretch and bend without breaking.

In fact, epoxy typically elongates to over 4%, while polyester only elongates to about 2-3%. This means that epoxy creates a material that’s much more resistant to the constant stress and strain that can occur in a boat.

Take a lightweight dinghy, for example. It’s constantly being subjected to dynamic loads from the water, the rigging, and the sails. If it’s built with polyester, the brittle nature of the resin can lead to microcracking. Over time, this can weaken the laminate and make it less durable.

But with epoxy, the increased flexibility helps it withstand these stresses better. And post-curing the epoxy can make it even stronger and more resilient.

So, the short answer is: no, epoxy is not more brittle than polyester. In fact, it’s quite the opposite.

Myth 5: Epoxy Won’t Cure In Cold Weather

I can understand where this myth comes from, although it’s still a myth at the end of the day. Our WEST SYSTEM® 105 Epoxy Resin with 205 Fast Hardener® will actually cure at temperatures of just 5°C (41°F) but it’s far from the optimal temperature for working with epoxy.

Just think of epoxy being a bit like honey. If you put it in the fridge it gets viscous and difficult to spread on your toast. In the case of epoxy you might not be able to wet out glass cloth quite so easily, or it might be harder to achieve the desired consistency when mixing epoxy with fillers. There’s also the risk that coatings go on too thickly, creating a subtle exotherm which can lower viscosity rapidly and increase the risk of running.

How can you work effectively with epoxy in cold weather?

The first thing you can try is warming up the epoxy (think of the honey example again). You can also create a tented area and use a small blow heater to increase the temperature where you’re working. That way the epoxy will be much easier to work with and you’ll get the best possible results and finish.

And the perfect temperature for working with epoxy? That has to be t-shirt temperature, meaning that if you’re comfortable working in a t-shirt you’ve got it right. For those who need a figure, let’s say 15°C!(59°F)

Myth 6: Epoxy Won’t Stick Underwater

This is definitely a myth. Some years ago we heard from a customer who tried to get rid of some left-over epoxy by flushing it down the toilet. It cured and stuck to the toilet pan and he eventually cracked the toilet trying to break up the epoxy with a chisel.

When it comes to emergency hull repairs and the like, we definitely wouldn’t recommend using epoxy below the waterline. However, WEST SYSTEM products might just buy you some time in the case of an emergency. We’ve actually had some success by applying some of our G/flex® 655 product to a fibreglass plate and carrying out temporary emergency repairs under the water.

For this reason, we think more of our customers should pack G/flex 655 epoxy and fibreglass sheets as part of their essential equipment on board. It’s one of those things that you hope you never need but it could be as vital as a pair of flares one day in an emergency situation.

Myth 7: Epoxy Is Only Good For Wood

Wrong! Whilst it’s true that WEST SYSTEM Epoxy was initially developed in the US for use with wood, it is a general purpose marine epoxy – and the product has been evolving constantly ever since.

Epoxy is now the material of choice for all manner of applications, from GRP builds and repairs, to fairing cement and aluminium hulls. It’s not just used for marine work either, we have customers who use our epoxy for repointing their houses, or for making decorations and even artwork with items set in cured epoxy. It’s even used on tube trains, buses, aeroplanes – the sky (or your imagination) really is the limit.
Just one caveat here is that different substrates need to be prepared differently to achieve great results with epoxy. We have a huge amount of literature on how to use epoxy with different materials – get in touch if you need support on a specific non-wood project.

Myth 8: Epoxy Won’t Stick To Plastic

False again! There’s a WEST SYSTEM product specifically for this and it’s called G/flex Epoxy. There are lots of reasons why it delivers better results with plastic than traditional WEST SYSTEM 105 Epoxy Resin.

First, I should say that you need to flame treat your plastic surface to get good adhesion with any epoxy product. By passing a flame incredibly quickly across the surface, without even heating the plastic, you create an oxidised surface that can be wetted out with the epoxy, rather than the plastic surface repelling the epoxy.
With the surface ready, G/flex Epoxy delivers amazing results. It is much more elastic than other epoxies, so it can flex with the plastic and tolerate shocks and impacts. Just as a comparison, a 10cm length of regular WEST SYSTEM Epoxy can stretch to 10.4cm before it breaks, while G/flex Epoxy will stretch to 13cm. It’s a huge difference and really important for strong, long-lasting results with plastic.

Myth 9: Epoxy’s Pot Life Is Too Short

It’s true that epoxy waits for no man, but planning effectively and working the right way can give you a longer working time.

One important thing to mention here is that we calculate a working time of 20 minutes based on mixing epoxy resin and hardener in a small paper cup. This is standard practice which allows you to compare working times between manufacturers, but you can buy more time by mixing your epoxy in official WEST SYSTEM® mixing pots, which are short and stout.

By reducing the depth of epoxy in the mixing pot, you also reduce the intensity of the exothermic reaction – which is why you get more working time. If you could mix your epoxy in a roller tray, you’d get even more time, but achieving a good mix would, of course, be impossible.

Another way you can extend your working time is by using WEST SYSTEM pumps. One pump of resin and one pump of hardener give you approximately 24 grams of mix, which is perfect for small jobs. With experience, you can work your way up to three pumps of each and still have time to work. As well as being the fastest way to measure out and mix epoxy, pumps help you work more cleanly, with no need to pour and weigh the epoxy resin and hardener.

Myth 10: Epoxy Is Harder To Use Than Polyester

When it comes to choosing a resin system, it’s definitely horses for courses and Polyester and Vinyl-Ester resins definitely have their place in boat building. Many times, professional boat builders have experience of working with both epoxy and Polyester and they find them both as easy to work with.

However, there are some reasons why epoxy might be a more convenient choice for amateur boat builders, DIY enthusiasts or anyone who hasn’t worked much with resin/hardener previously.

First, Polyester often comes in large packs, which might not be convenient or cost effective for all users. Second, WEST SYSTEM pumps make epoxy easier to measure out and mix, with no need to weigh Polyester and catalysts. Thirdly, inexperienced users may find Polyester more pungent than epoxy, which could be off-putting, especially if you’re working in a confined space.

So no, epoxy definitely isn’t harder to use than Polyester and it might even be easier for some people who are inexperienced working with different resin systems.

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

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

Looking for guidance and tips on how to use epoxy products for your boat build? Have a look at our full list of Trade Secrets.