Epoxy mix ratios and exotherm: why "a bit more hardener" ruins a pour

Epoxy is unusual among the materials hobbyists work with, in that it gives you almost no room to improvise. Add extra hardener to speed it up, as you might add extra catalyst to polyester resin, and you do not get a faster cure — you get a batch that never cures properly at all. Understanding why makes the rest of the rules obvious.

The ratio is chemistry, not a recipe

Epoxy cures by a reaction between two different molecules: the resin, carrying epoxide groups, and the hardener, typically an amine carrying reactive hydrogen sites. Each resin site pairs with a hardener site to build a single large cross-linked network. That is what turns a liquid into a rigid solid.

The ratio a manufacturer specifies is the one that supplies roughly matching numbers of these sites. Deviate, and the surplus component has nothing left to react with. It does not evaporate or eventually harden; it stays in the network as unreacted material, permanently. The result is the classic failed pour: soft or tacky patches, reduced strength, sometimes a cloudy finish, and in bad cases a batch that never hardens at all. There is no rescue for a genuinely off-ratio pour — it has to come off and be redone. Worse, a mix can harden and still be defective: an off-ratio batch that feels solid can have lower heat resistance, chemical resistance and strength than it should, which matters for anything structural.

Before assuming the worst, check for amine blush. A greasy or waxy film on an otherwise properly cured surface is usually not a ratio problem at all. It is a by-product formed when the amine hardener meets moisture and carbon dioxide in the air during cure, and it is especially common in cool, damp conditions. Blush is harmless and water-soluble: warm soapy water and an abrasive pad remove it, where solvents largely do not. It does have to be removed, because nothing — another coat of epoxy, varnish or paint — will bond properly over it. Scraping off a sound pour because its surface felt waxy is a genuinely expensive mistake.

Polyester resins work differently, which is where the dangerous habit comes from. Their MEKP is usually called a catalyst, though strictly it is an initiator — it starts a free-radical chain reaction and is consumed doing so, which a true catalyst would not be. Within its specified range, the amount mainly changes the speed rather than the final network, so fibreglass workers learn to dose for the weather. That range still has limits in both directions: too little leaves a permanent undercure, too much makes the casting brittle and prone to cracking. Carry the habit over to epoxy, where the ratio is stoichiometry rather than dosage, and it fails outright. If you want a faster epoxy cure, change the hardener grade or the temperature, never the ratio.

Weight and volume ratios are different numbers

A ratio only means something when you know what is being measured. Resin and hardener usually have different densities, so the same mix expressed by volume and by weight gives different figures — a system that is 1:1 by volume might be roughly 100:83 by weight. (That figure is an illustration of the size of the gap, not any particular product's specification: read your own data sheet.)

Manufacturers publish one or both, and the two are not interchangeable. Mixing a weight ratio using measuring cups, or a volume ratio on a scale, is a common and expensive mistake. Check which one your product's data sheet states, and measure in that unit.

Weight is generally the easier of the two to get right: a digital scale reading in single grams beats eyeballing graduated cups, particularly for small batches. Volume measuring is workable, but it needs cups with genuine graduations and enough patience to let thick resin settle and drain.

Tolerances are tighter than people expect. Many systems want accuracy within a few percent, and small batches are where errors creep in — 5 g of error matters enormously in a 50 g mix and not at all in a 5 kg one. If you need a tiny quantity, mixing a slightly larger batch and discarding the excess is often more reliable than trying to weigh two grams of hardener accurately.

Mixing is part of the ratio

A correctly measured batch that is poorly mixed behaves like an incorrectly measured one, because locally the ratio is wrong. Scrape the sides and the bottom of the container thoroughly — unmixed resin clinging to the wall is the usual culprit behind a sticky patch in an otherwise good pour. Many workers transfer the mix to a second clean container partway through and continue mixing there, which leaves any unmixed film behind in the first cup.

Mix deliberately rather than vigorously. Whipping air into the resin creates bubbles you then have to remove, and heat from friction shortens your working time.

Your skin: the hazard most guides leave out

Uncured epoxy is a sensitizer, and this is the most common way hobby use causes lasting harm. The distinction that matters is between irritation and allergy.

According to WEST SYSTEM, one of the larger manufacturers, "fewer than 10% of epoxy users react when overexposed to epoxy resin or hardener", most commonly with contact dermatitis — skin inflammation that clears up once exposure stops. Fewer than 2% develop allergic dermatitis, and that one does not clear up. Their description of how it arrives is worth quoting exactly: "You may become sensitized to epoxy after many exposures or just one. It could take ten days of exposure, a month, or even years." After that, "additional (and sometimes increasingly severe) reactions become likely upon future exposures, even to tiny amounts of epoxy."

There is no cure and no tolerance to build. People have had to give up the hobby, or the trade, over it. All of which makes the precautions disproportionately worth taking for how dull they are:

Wear impermeable gloves and do not touch uncured resin or hardener. The amine hardener is the more aggressive half. Change gloves when they are contaminated rather than working on in them.
Never wash resin off your skin with solvent. Acetone and similar strip the skin's barrier and help the chemistry through it. Soap, water and a waterless skin cleaner are the right answer.
Cover up and ventilate — long sleeves, and air movement when working with any quantity, particularly in a small space.
Never sand epoxy that is not fully cured. The manufacturer's wording is blunt: "Never breathe the sanding dust of partially cured epoxy. Serious health problems can result." Partly cured dust is still chemically reactive, and it lodges where you least want it. Let it cure fully, and use dust extraction and a mask even then.

Exotherm: the part that can actually be dangerous

The curing reaction is exothermic — it releases heat. In a thin film, that heat dissipates into the air and the substrate and nothing remarkable happens. In a deep pour or a large mass left in the mixing cup, the heat cannot escape, and it accelerates the reaction, which releases more heat, which accelerates it further.

A runaway exotherm can produce a great deal of heat quickly. In practice this shows up as resin that yellows, cracks internally, shrinks away from the mould, smokes, deforms its container, or in bad cases becomes hot enough to be a genuine burn and fire risk. It is the single most common way a hobby resin project goes seriously wrong.

Three things drive it: mass, depth, and the formulation. That is why:

Deep pours need deep-pour resin. These systems are formulated to react more slowly, and their instructions specify a maximum pour depth — often in the range of a few centimetres per layer. General-purpose coating epoxies can have maximum depths of only a few millimetres.
Layering beats one big pour. Let each layer cool and cure enough before adding the next.
Never leave a mixed batch sitting in the cup. A full cup of standard epoxy is a compact mass with nowhere to dump its heat; poured out thin, the same quantity is harmless. If you must abandon a batch, spread it out on something non-flammable and let it cure outdoors or somewhere ventilated. Never drop a hot mass into a bin containing paper, sawdust or solvent-soaked rags — that is how a runaway becomes a fire.
Mind the container itself. Don't mix large masses in glass, which can shatter under the thermal shock, and be aware that thin plastic cups soften and slump when a batch runs hot.

Warmer ambient temperatures make all of this more energetic, and cold slows curing down to the point where some systems will not cure at all below their minimum working temperature.

Working out how much you need

Coverage for a flat pour is a volume calculation: area × thickness. A square metre at 1 mm thick is one litre, which scales conveniently — 2 mm over half a square metre is the same litre.

Epoxy is usually sold and often mixed by weight, so you will need to bridge the two: cured epoxy runs around 1.1 g/cm³, which puts a litre at roughly 1.1 kg. Ignoring that is a common way to under-buy.

For moulds, water is a reasonable proxy for volume if the mould is sealed — but weigh the water rather than reading a jug, remember it is lighter than the resin that will replace it, and dry the mould completely. Residual moisture causes cloudiness and surface defects, and "thoroughly" here means bone dry, not towelled off.

Mix a margin over the calculated figure. Resin left in the cup and on the stirrer is unavoidable, and running short mid-pour is worse than a little waste, because a second batch poured onto a partly gelled first one can leave a visible line.

Getting the numbers right

None of this needs software: the ratio in whichever unit your data sheet states, area × thickness for coverage, and about 1.1 kg per litre to convert. What it does need is a scale that reads in single grams and the discipline to use it for every batch, including the small ones.

The data sheet for your specific product always wins, though. Ratios vary by system, and the one printed on your bottle is the one that matters.