How to Mix Epoxy Resin A and B for Concrete Floor Paint: A Field Checklist

I review epoxy products for a living. I sit on the quality side of a chemical plant—checking resin batches, certifying shipments, and investigating field failures when customers call to say their concrete floor epoxy paint never cured. I'm not a flooring installer. But most of those calls have one thing in common: the mixing process went wrong, not the chemistry.

If someone asks me how to mix epoxy resin A and B in one sentence, my answer is: measure the two components according to the product data sheet, mix until completely uniform, and apply before the pot life expires. The checklist below fills in the parts that most articles skip.

Not all two-part resin floors are epoxy. If you are looking for BDO for PU resins as a chemical intermediate, that is a manufacturing conversation, not a floor-coating conversation. This article applies to two-part epoxy systems only.

Before the Checklist: Know What You Are Working With

Concrete floor epoxy paint is usually a two-component product. Part A is a liquid epoxy resin. Part B is a hardener. When they are mixed in the correct ratio, the chemical reaction turns the liquid into a tough, continuous film. When the ratio is wrong, or the mixing is incomplete, the floor may cure soft, stay sticky, blush, or fail in patches.

For context: I work at Olin, and Olin epoxy resins are supplied to coating manufacturers around the world. In older records you will sometimes see the company name Olin Mathieson Chemical Corporation; that history still matters in industrial chemicals. But even with a well-made resin, the installer's mixing method decides whether the final floor performs.

The 7-Point Mixing Checklist for Epoxy Resin A and B

1. Read the technical data sheet before you open anything

The TDS contains the exact mix ratio, recommended temperature, mixing time, working time, and cure schedule for that specific system. The label on the pail is usually the shorter version of the same information. Don't rely on memory—even for a product you have used before. Formulations can change.

2. Bring both components to the right temperature first

Cold epoxy resin is thick. It pours poorly, mixes poorly, and makes people think they need to add solvent or heat. Most concrete floor epoxy paint products are designed to be applied at 18–24°C, which is roughly 65–75°F. Move the pails into a warm room a day ahead if you can. Do not aim a propane torch or heat gun directly at the pail.

3. Mix Part A by itself before adding Part B

Part A is not always a simple clear liquid. Pigments can settle. Fillers can sink to the bottom of the drum. If you skip this step, the first part of your floor may look lighter, darker, or blotchier than the rest. Stir Part A until it is uniform before you ever pour Part B into the mix.

4. Measure by weight unless the data sheet says otherwise

Here's the thing: many epoxy systems are weighed by parts per hundred resin, not measured by volume. A mix ratio like 100:43 means 100 parts of resin and 43 parts of hardener by weight. If you measure the hardener by volume instead, you can be off by a surprising amount because hardeners often have different densities.

Use a digital scale. Tare the bucket, add the correct weight of Part A, then add the correct weight of Part B. If the manufacturer gives a volume ratio, follow that. But if the TDS says by weight, do not switch to cups because it is faster.

5. Mix slowly, and mix for the full specified time

Once Part B is poured into Part A, the clock starts. Mix at a slow to medium speed with a paddle mixer. Most products need two to three minutes of active mixing. Stop the drill at least once, scrape the sides and the bottom of the bucket, then mix again.

Look at the mixture before you use it. It should be a single uniform color with no streaks, swirls, or clear pockets. Some epoxy hardeners are dark amber, which makes color checking less obvious, so slow down and look closely.

6. Transfer to a clean roller tray or bucket

This step sounds fussy. It isn't. A mixing paddle can throw material up the sides of the bucket, and that material may stay unmixed. If you pour straight out of the mixing bucket onto the floor, those unpainted? patches of unmixed resin can end up as soft spots in the cured coating. Transferring to a clean container is a simple way to avoid a very unhappy phone call.

7. Work inside the pot life

Pot life is not a suggestion. For many concrete floor epoxy paint products at room temperature, you have somewhere between 20 and 60 minutes after mixing before the material starts to thicken. Larger mixed batches shorten the pot life because the chemical reaction gives off heat.

If the epoxy becomes stringy, lumpy, or warm to the touch, stop. Do not add solvent. Do not add extra hardener. That batch is done. Get a fresh batch and adjust the size of your mix.

Field Mistakes I Keep Seeing

I have mixed feelings about calling these common mistakes because most of them come from people who are trying to get the job done quickly. But the consequences are real.

Adding extra hardener to make it cure faster

This is the biggest myth I run into. Extra Part B does not speed up the cure in a useful way. It changes the chemical ratio, and the floor can end up soft, greasy, or vulnerable to moisture. A two-part epoxy that is mixed incorrectly is not still the same product—it is a different, worse formulation.

Using Part B from another kit

I once investigated a floor failure where the crew had used extra hardener from a similar-looking pail. The hardener was not defective. It was simply a different hardener with a different mix ratio, and it was never designed to work with that resin. The floor cured unevenly, and the whole coating had to be removed.

Mixing a batch that is too large

Epoxy generates heat as it cures. If you mix a five-gallon pail but only need one gallon, the excess material can exotherm in the bucket and harden into a dangerous, hot block. Mix only what you can apply within the pot life for your application method.

Painting over damp or dirty concrete

A perfect epoxy mix will not bond to a damp, dusty, or contaminated slab. If you are working over concrete, check for moisture before you pour anything. A simple plastic-sheet test per ASTM D4263 can catch obvious moisture problems. Larger projects should use a qualified concrete moisture testing method. I am not a concrete specialist, so I will leave the details to the slab experts—but I know that no resin can fix a bad substrate.

Final QA Before You Start

Before mixing, ask yourself these questions: Is the concrete clean and dry? Is the surface profile right? Are the air, material, and substrate temperatures within the ranges on the data sheet? What is the dew point? The quality of the floor depends on those environmental conditions just as much as the ratio.

Record the batch numbers of both components, the mix date, and the environmental conditions. If something does go wrong later, that information is gold. In my job, a call with a technical data sheet and batch number is far more useful than a vague description of what was used.

One last reminder: let the floor cure before you abuse it. Being able to walk on a coating after 24 hours does not mean it is ready for forklift traffic. Full chemical and mechanical resistance often takes several days. Check the product data sheet for the specific concrete floor epoxy paint you purchased.

Mixing epoxy is not complicated, but it is unforgiving. Measure correctly. Mix completely. Trust the data sheet. That is the whole trick—and it is the difference between a floor that looks good for years and one that fails before the paint smell disappears.