Mixing, Ratios and Pot Life
Two Parts, One Fixed Ratio
Epoxy is a thermoset. Part A, the resin, carries epoxide groups. Part B, the hardener, is usually an amine that reacts with those groups. Each molecule of hardener is designed to link a specific number of resin sites, which is why the manufacturer states a ratio by volume or by weight and treats it as fixed. Two to one, three to one and one to one are all common, and they are not comparable across products.
Adding extra hardener does not make the floor cure faster or harder. It leaves unreacted amine in the film, which softens it, causes blush and can leave a permanent surface tack. Short changing the hardener leaves unreacted resin, which stays soft and gummy and never reaches its stated chemical resistance. Neither error corrects itself and neither can be sanded out, so the only real fix is removal.
The stated measure matters too. A product specified by weight and mixed by volume will be off, because resin and hardener rarely share a density.
Induction Time
Some products call for an induction period, sometimes called sweat in time: mix the two parts, then wait a stated number of minutes before applying. That pause lets the initial reaction begin so the material has the right viscosity and wetting behaviour when it hits the concrete. Skipping it where required can leave the coating too thin to build film.
Just as many products say not to induct, because the clock is already short. Induction is a per product instruction, not a general technique, and applying it from habit is a common way to lose a batch.
Mixing Properly, Not Merely Stirring
Hand stirring with a stick does not mix two part epoxy. The materials are viscous and they need mechanical shear from a drill with a proper mixing paddle, typically at a few hundred revolutions per minute for two to three minutes. Fast enough for full incorporation, slow enough to avoid whipping air into the film.
- Scrape the sides of the pail, where a band of unmixed resin always clings.
- Scrape the bottom corner, where hardener collects and the paddle does not reach.
- Many crews box the material: mix in the first pail, pour into a second, mix again briefly. Anything left unmixed in pail one is left behind.
Unmixed material that reaches the floor shows as soft spots, glossy patches that stay tacky, or small areas that never harden. They are usually only a few centimetres across, and they are permanent.
Pot Life and Exotherm
The reaction is exothermic: it produces heat. Once the two parts are combined, the mixed material warms itself, and because the reaction speeds up with temperature, that heat accelerates the reaction, which makes more heat. In a deep pail the mass has nowhere to shed it, so a batch sitting in a bucket runs away far faster than the same material spread thin on the floor.
Pot life is the working time in the container and it is always shorter than the working time on the ground. A product quoting thirty minutes of pot life at 25 C may give considerably less on a hot Allen afternoon when the material is already warm when it comes off the truck. A neglected pail will get hot enough to be uncomfortable to hold, will begin to smoke, and will set into a solid block that has to be thrown away with the bucket. An overheating pail goes outside immediately rather than being left in a closed garage.
Batch Sizing to the Crew and the Area
All of this makes batch size a planning decision rather than a convenience. The crew mixes what it can spread and back roll before the pot life ends, given the number of people, the layout of the space and that day's slab temperature. On a warm day that often means smaller, more frequent batches, and the material getting out of the bucket and onto the floor promptly, where the thin film sheds heat and the working time extends.
It also means the space is prepared before anything is mixed. Once the paddle goes in, the clock is running.