We asked Hamish: why isn’t my epoxy curing?


It’s one of the most common questions: why isn’t my epoxy curing? Fortunately, when it comes to WEST SYSTEM® epoxy products, it’s quite easy to work out precisely what’s happening. We spoke to our technical guru Hamish Cook to find out what can go wrong and how avoid it happening again.

“Why isn’t my WEST SYSTEM epoxy curing?” I regularly get emails asking me this question. Certainly, when you’ve been working hard on a project it’s deeply frustrating to realise that your components are not bonding, or that your lamination isn’t curing.

Fortunately, there are three common reasons why this happens.

1. The resin and hardener are not mixed at the correct ratio

This is arguably the most common reason of all. Mix ratio is so important and if it’s wrong your epoxy mix will not cure.

For certain blends – as outlined below – there will be a difference in your mix ratio depending on whether you’re mixing by weight or by volume. Always check what it says on the packaging before you start but just to reiterate:

• If you’re mixing WEST SYSTEM 105 Epoxy Resin® and 205 Fast Hardener® or 206 Slow Hardener®, the ratio is five to one: that’s five parts resin to one part hardener. This is correct whether you’re measuring by weight or by volume.
• If you’re mixing WEST SYSTEM 105 Epoxy Resin® with 207 Special Clear Hardener® or 209 Extra Slow Hardener®, you need three parts resin with one part hardener by volume. By weight, you need three-and-a-half parts resin to one part hardener.
Someone once told me that they’d mixed their resin and hardener at the wrong ratio and asked: could they just brush more hardener across the top? Unfortunately the answer is “NO!” If the ratio isn’t correct, there’s nothing for it but to start again with a new mix.

Blending your resin and hardener is also really important – see the next point.

2. The resin and hardener are not blended enough

If you’re sure you mixed your resin and hardener at the correct ratio but your epoxy mix is still not curing, then it may be that you haven’t mixed it enough.

All of our literature recommends that you sit and mix your epoxy resin and hardener for two minutes; that’s quite a long time if you think about it. However, it’s important to do this thoroughly to get all the resin and hardener off the sides/the bottom/out of the corners of your mixing pot. Using a scraper with a flat bottom is good for this.

3. It’s the wrong hardener for the working temperature

Using the correct hardener for the working temperature is also important.

The 206 Slow Hardener needs a working temperature of 16°C to cure. So if you’re using this hardener and your epoxy blend isn’t curing, then it may just be a case of allowing more time; it probably will cure eventually.

Our 205 Fast Hardener, meanwhile, is the most versatile and will cure down to 5°C . However, do bear in mind that the fast hardener is slower in colder temperatures. So, as we enter the winter months, it may take a little longer than usual to go off.

Pot life vs cure time

It’s also important to understand the difference between cure time and pot life. The pot life of a standard 105/205 blend is about fifteen minutes. However, that’s not the same as the cure time.

When a mass of blended epoxy resin and hardener is contained in a pot, it generates heat and cures quickly. Yet if you pour this blend out into a tray, you’ll extend its working life by about five minutes. Brush it as a thin coat onto ply and it’ll take anywhere between one and two hours to cure.

So one reason that your epoxy blend hasn’t cured may be that it wasn’t meant to – at least, not yet! Give it a couple of hours and you’ll know for sure.


Our thanks to Hamish Cook for his contribution.

Still having trouble working with your WEST SYSTEM epoxy products? Have a look at our Problem Solver.


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