Craft & DIY

#20 · Mix resin or epoxy

Stir two-part epoxy or casting resin with a disposable chopstick, then throw it away guilt-free. No brush ruined.

Anyone who has tried casting resin knows the failure mode: a piece that looks perfect, and is still tacky three days later. Almost every time, that is a mixing problem rather than a product problem, and a chopstick is the disposable stirring tool the job actually wants.

Epoxy cures by chemical reaction between two components in a fixed ratio. It does not dry. Any resin molecule that never meets a hardener molecule stays liquid forever, which is why 'mostly mixed' is not a thing. The tacky patches on a failed cast are usually a perfect map of where the stirrer never reached.

How to do it

Measure both parts precisely, by weight if the manufacturer specifies weight and by volume if it specifies volume. These are not interchangeable.
Stir slowly. Fast stirring whips in air bubbles that you will then spend an hour trying to remove.
Use a figure-of-eight rather than circles - circles can rotate the whole cup without shearing the two liquids together.
Scrape the sides and the bottom repeatedly. Unmixed hardener clings to the cup wall, and this is the single biggest cause of sticky castings.
Stir for the full time on the packet, usually three to five minutes. It always feels like too long. It isn't.
Decant into a second clean cup and stir again for a minute - the professional trick that eliminates the wall-cling problem entirely.
Pour, then bin the chopstick.

Why it works

Resin and hardener are viscous, so they don't diffuse into each other on their own - you have to physically fold them together, which is what shearing means. The corner where a cup's wall meets its base is a dead zone that a curved stirrer never properly reaches, and double-potting works because the second cup has no unmixed film on it.

If it isn't working

Tacky patches on the surface: incomplete mixing, or the ratio was off. Neither is fixable after the fact - sand it back and re-pour.
The whole piece stays soft: measurement error. Check whether your resin wants weight or volume.
Cloudy or bubbly: stirred too fast, or the resin was cold. Warm the bottles in a bowl of hot water before mixing.
It cured in the cup before you poured: too large a batch. Epoxy generates its own heat, and volume accelerates it.

Tips & warnings

Never reuse a resin chopstick, even one that looks clean.
Use plain bamboo, not lacquered - flakes of finish in a clear casting are very visible.
Two chopsticks are useful: one to stir, one to scrape the first one clean into the cup.