Epoxy Basics: How much epoxy resin do I need?

This is a question the team at West System International get asked a lot. According to technical expert David Johnson, it’s important to have a good idea of how much epoxy your project is going to need before you start. You want to be sure you have enough to hand for the job – but equally, you don’t want to spend money on epoxy that you don’t need.

Fortunately – thanks to many, many years of working with epoxy – we’ve devised a simple and reliable way to work it out.

Let’s start with our three rules of thumb about coverage:

For the first coat onto porous wood, the epoxy coverage rate is 6m² per kilo.
After the first coat, the epoxy coverage rate is 8m² per kilo.
With fibreglass boats, you work on a 1:1 resin to fibre ratio. So if you buy 200g of glass cloth, you’ll need 200g of epoxy to wet that out.

Note that these weights are inclusive of hardener.

How do I work out the surface area?

This is a case of going back to school maths!

You can use the waterline length and beam to create an approximate surface area. It’s a very good guide.

Generally when you’re coating the underwater area of a boat, you can use the waterline length and the beam to create an approximate surface area. It’s a very good guide, and it works for a lot of different shapes.

Of course, many modern boat plans are produced by CAD, and cut with CNC cutters for accuracy. Ask the designer the surface area, and they’ll tell you to the nearest detail.

Some fibreglass boats will have a long moulded-in keel. That’s harder to estimate, but this is where the team at West System International can help you out.

We know just about every boat in production. If you’re stuck, tell us about yours and we can make an educated guess about the surface area.

What epoxy pack size should I choose?

Here, it’s all about economies of scale.

Someone that needs exactly 4.5kg of epoxy is better off buying a B pack which has six kilos of resin and hardener than, say, four A packs. It’s more cost-effective per kilo. And it offers a little bit extra that they can use for something else.

Our thanks to David Johnson for his great insight and tips.

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