The right ratio isn’t a suggestion
The resin contains epoxy groups, and the hardener contains amines. Only when they meet in the right ratio do the epoxy rings open and the molecules link up into a solid plastic. Every epoxy group needs a reaction partner from the hardener’s amine groups. That’s why the “right ratio” isn’t a suggestion. It’s a chemical necessity.
Why measuring a weight ratio by volume is wrong
Resin and hardener have different densities. A given volume of resin weighs more than the same volume of hardener. If you measure a weight ratio by volume, you end up with too few amines for your epoxy groups at the molecular level, or the other way around. The result: many molecules never find a reaction partner.
Too much hardener or too much resin
Too much hardener: the surplus amines can’t find enough epoxy groups to react with, so they stay unbound and disrupt the structure. The piece stays flexible and bendable and doesn’t reach the intended strength. The unreacted amines stay in the piece and can still irritate or sensitize your skin.
Too much resin: many epoxy groups stay unbound, and the reaction never runs to completion. The surface stays tacky, the material doesn’t fully cure, and it never reaches its intended strength. Chemically, the piece remains permanently undercured, and time won’t fix it. The unreacted epoxy groups can still sensitize your skin, too.
The right scale
You don’t need complicated lab equipment, but a digital precision scale is a must. For small projects, use a scale that reads to about 0.0035 oz (0.1 g); for larger pours, about 0.035 oz (1 g) is fine. If you work by feel or by eye, you risk sticky, soft, or unstable results. Once resin and hardener meet, there’s no going back.
Then mix thoroughly
Even precise weighing only pays off if you mix well. A proven approach is the 3-3 mixing method: stir for the first three minutes with even strokes, in a spiral through the mixture, and again and again along the bottom and sides. Then transfer the mixture to a fresh, clean cup and stir for another three minutes. Keep in mind that the working time starts the moment you mix resin and hardener.
Frequently asked questions
Can I mix epoxy resin by eye?
No. If you work by feel or by eye, you risk sticky, soft, or unstable results. For small projects, use a digital precision scale that reads to about 0.0035 oz (0.1 g); for larger pours, about 0.035 oz (1 g) is fine.
What happens if I use too much hardener?
The surplus amines can't find enough epoxy groups to react with, so they stay unbound and disrupt the structure. The piece stays flexible and bendable and doesn't reach the intended strength. The unreacted amines stay in the piece and can still irritate or sensitize your skin.