Trade Secrets: TRADE SECRET: Thinning epoxy: Best practice guidance for reducing the viscosity of WEST SYSTEM epoxy
How to thin your epoxy and what to consider before you do
Reducing the viscosity of WEST SYSTEM® epoxy by adding a solvent, can be tempting. It becomes easier to roll out; it penetrates more deeply into porous surfaces such as partially rotten wood; and it saturates faster into materials like fibreglass fabric.
But – and it’s a big ‘but’ – reducing viscosity can also change the characteristics of the epoxy, impacting both its strength and moisture resistance. It also affects the time it takes for the epoxy to gel.
So, before you decide to thin your epoxy, it’s important to weigh up the benefits against the potential risks. Then, if you want to go ahead, you need to decide whether to thin your epoxy using heat or with a solvent.
Some common misconceptions about thinning epoxy
The technical staff at West System International often speak to customers who believe that epoxy needs to penetrate deeply into wood to be effective. Although that is sometimes the case, most of the time it’s not – and customers are thinking of thinning epoxy when it might not actually be beneficial.
A neat epoxy coating on the surface, for example, provides far better water resistance than thinned epoxy, because thinned epoxy tends to become porous. Similarly, adhesion depends on the surface area of a joint, the strength of the wood and the strength of adhesive used and not – as many believe – on deep penetration of epoxy. And when it comes to rotten wood, although thinned epoxy certainly makes it harder, it doesn’t regain its original strength.
So if you’re thinking of thinning epoxy for any of these reasons, it might be time to think again. Otherwise, here’s what you need to know to thin epoxy effectively:
You have two options if you want to thin epoxy using heat. You can heat the resin and hardener components separately and then mix them together to create a thinned epoxy. Or you can heat the substrate – such as wood – and apply your room temperature resin and hardener mix to the heated surface.
A heated mix of resin and hardener does retain all the characteristics of epoxy cured at room temperature – but it cures faster, which might take you by surprise. Using WEST SYSTEM 206 Slow Hardener® or WEST SYSTEM 209 Extra Slow Hardener® can help, as these two hardeners have a slow cure speed giving you more time to work with the warm epoxy.
But if you’re coating wood, the best method is to warm the wood, rather than the epoxy. Remove the heat source just before applying epoxy and the mix will thin as it comes into contact with the wood. As the wood cools, the epoxy is then drawn in deeply before gelling, as the air in the wood fibre contracts.
Whichever method you use, you should be able to comfortably touch the epoxy containers or the wood, so heat them to a maximum of 35˚C. Overheating causes the epoxy to harden too fast. If you see smoke rising from the cured epoxy, it is probably damaged and needs to be replaced.
Whereas heating epoxy can enable you to maintain the original epoxy characteristics, adding a solvent such as acetone, lacquer thinner or denatured alcohol can create drastic changes. We do not recommend thinning epoxy with solvent at any time, and here’s why:
- Adding 5% lacquer thinner to epoxy reduces epoxy’s compressive strength by 35%. As such, it’s no longer suitable as a structural adhesive.
- Adding a solvent can extend curing time, making your work unpredictable.
- Adding a solvent can cause the epoxy to shrink and crack over time. This happens if the solvent doesn’t evaporate before the epoxy hardens, but then finds its way out of the mix over time.
- Adding a solvent such as acetone can change the colour of cured epoxy.
- Adding a solvent can damage substrates such as Styrofoam – so test your solvent on your substrate before thinning your epoxy.
- Adding a solvent can increase the risk of fire and damage your health.
- Adding a solvent to wet out fibreglass quickly can create resin-starved fabric if excess epoxy drains out of vertical surfaces.
Overall, our recommendation is to thin epoxy using heat, because it can enable you to maintain the original characteristics of the epoxy.
But this recommendation always comes with a caveat: by thinning epoxy, you’re altering the composition of WEST SYSTEM products that have been refined over many years. That’s not to say you should never do it, but it does mean that you need to think and act carefully when you do.
Want to find out more? Read our guide to using WEST SYSTEM epoxy products.