Epoxy Not Curing? The Science of Thickness, Heat and Mass
Why thickness, heat and mass decide whether your repair cures
Mixed and ready to apply. Fold the 4 Hour until it's one uniform golden colour, with no streaks and A correctly mixed, correctly applied repair curing on a window sill.
Left: the 4 Hour mixed to an even golden colour, ready to apply. Right: Evermend curing on a window sill.
This morning we got a phone call from a customer. They had applied Evermend 4 Hour Cure on Friday afternoon. They came back on Monday and the repair hadn't cured. It was still tacky, days past the four hours on the label. One thing is for certain: if your epoxy has not cured after three days, it probably never will.
Why?
It's a good question and this is a very frustrating situation. However, epoxy is a chemical reaction, and it depends on a handful of conditions.
The most common culprits are the ones you'd expect: an off-ratio mix, incomplete mixing, or damp and cold conditions.
But there's another factor that catches people out more than any of these. The amount of epoxy in a repair, and specifically how thick it is, changes how fast it cures. A thicker repair can cure faster than a thin one, because epoxy doesn't dry. It reacts, and the reaction produces its own heat.
That heat is the subject of this article. We'll explain what's actually going on, when to use the 1 Hour and when to use the 4 Hour, and what to do if your repair hasn't cured.
If you're reading this because a repair has gone wrong, skip to section 8 for a step-by-step checklist. And if you're stuck, call us.
The specs at a glance
Evermend 1 Hour | Evermend 4 Hour | |
Cure time at 20°C | 50-60 minutes | 4 hours |
Ready to sand and paint | 1 hour | 4 hours |
Application thickness | 5-15 mm | 5-20 mm |
Application temperature | 0-25°C | 0-35°C |
Typical uses | Spot repairs, splits, splicing, small repairs | Larger repairs, large projects, timber resin repair |
Both products start at 5 mm. Neither is designed for thin skims, and the reason is the subject of this article.
1. What "curing" actually is
Evermend is a two-part epoxy: a resin (the base) and a hardener (the activator). When you mix them, the molecules in each part react and link together into a tightly connected network. That network is the rigid, structural material you sand and paint.
The hardener isn't a catalyst that simply sets things off. It gets used up and becomes part of the finished material.
That's why the 3:1 ratio matters: it balances the two parts so nearly every molecule finds a partner. Too much or too little hardener leaves some molecules with nothing to link to, so the repair stays soft, however long you wait. It's also why adding extra hardener to "speed things up" backfires. It throws the balance off rather than speeding anything up.
Every link that forms also gives off a little heat. One gives off almost nothing, but a whole repair full of them adds up, and that heat is the key to everything else in this article.
2. The exothermic feedback loop
Reaction rate depends strongly on temperature. Combine that with heat release and you get a loop:
The reaction releases heat.
The mass warms up.
The warmer mass reacts faster.
The faster reaction releases heat faster, and the loop repeats.
So epoxy cures largely by heating itself.
3. The heat balance: thickness decides
Heat generated is proportional to volume. Twice the epoxy means twice the reactions and twice the heat.
Heat lost is proportional to exposed surface area. Heat escapes through the surface facing the air, and into the timber, at a rate that depends on the temperature difference.
Take a repair with one exposed face. The amount of epoxy sitting behind each square centimetre of that face is simply its thickness:
It's thickness that matters, not area. A wide, thin repair loses heat across its whole face just as a narrow, thin one does. A large area at 3 mm is still a thin repair.
Damp or cold timber makes it worse.
4. "That sounds backwards": why more epoxy cures faster
If you're thinking this makes no sense, surely more product takes longer to cure, it's the most common source of confusion with epoxy.
Our instinct comes from paint, filler and glue, which dry. They lose water or solvent to the air, and a thin layer dries faster than a thick one because the moisture has less distance to travel. Epoxy doesn't dry. It reacts, with no solvent to evaporate, and the reaction generates its own heat.
So the "4 hours" on the label is how long a suitable mass, at a suitable temperature, takes to complete its reaction.
Same 4 Hour product | What happens |
20 mm fill in a warm, dry room | Self-heats, and cures close to the stated time or faster |
20 mm fill in cool conditions | Slower, but still gets a self-heating boost |
5 mm fill in a warm room | Close to the stated time, slower at the thin end |
2 mm skim, cold and damp | May not cure at all |
5. Why the spec has a floor, a ceiling and two different temperature limits
The published ranges tell you where each product's heat balance works.
The 5 mm floor (both products). Below about 5 mm, the epoxy loses heat faster than it can build it, so the cure follows the ambient temperature, which is often too cold for a reliable result. The 5 mm floor is where self-heating reliably comes into play.
The 15 mm and 20 mm ceilings. Above the maximum, a single application holds so much heat that the loop runs away. Cure accelerates and can overheat, gel before you've finished shaping, or discolour. The 4 Hour is a slower-reacting formulation, so it tolerates a thicker section (20 mm) than the faster 1 Hour (15 mm).
The 25°C vs 35°C upper temperature limits. A faster formulation has less headroom. On a hot day, the extra ambient heat pushes the 1 Hour towards curing before you're ready, so its recommended ceiling is lower. The 4 Hour is designed to give you working time on warm days, so it goes to 35°C.
A caution on the temperature range: both products can be applied from 0°C, but that's the range in which they'll cure, not the range in which they'll cure at label speed. The 4-hour and 1-hour figures apply at 20°C. Colder than that, allow longer.
6. Two masses: the pot and the repair
There are actually two lumps of epoxy in play, and both self-heat.
The mass in the repair, which determines whether it cures reliably. This is what the thickness range is about.
The mass on the mixing board or in the pot, which determines how long you have to work. Mixing triggers an exothermic reaction, so larger batches cure faster and generate more heat, and mixed product shouldn't sit on the board.
This is why the 1 Hour and 4 Hour can both take a 5-15 mm fill, yet suit different jobs. A big batch of 1 Hour, mixed in one lump, heats and cures quickly, which is fine for a small repair where you can work quickly and a problem for a large one. A big repair needs long working time and a slower, steadier heat release. That's the 4 Hour.
7. Which product should I use?
Since both start at 5 mm, choose by size of job, working time and conditions.
Choose the 1 Hour for small repairs, spot repairs, splits and splicing, and jobs where you want to sand and paint within the hour: an urgent fix on a door or window, for instance, or a small repair on a cooler day.
Choose the 4 Hour for larger repairs and larger projects. That means deep rot repairs to window frames, door sills and bottom rails, big sections of decayed timber, structural joints, detailed profiles you need time to tool, and vertical or overhead work where the non-sag formula holds its position. Also choose it on warm days, or whenever you need to work for more than a few minutes.
1 Hour | 4 Hour | |
Job size | Small | Large |
Thickness | 5-15 mm | 5-20 mm |
Working time | Short | Long |
Hot days (over 25°C) | Outside recommended range | Suitable up to 35°C |
Big batch | Heats and gels fast | Built for it |
Ready to sand and paint | 1 hour | 4 hours |
In the grey area:
Check the depth. Under 5 mm, neither product is in spec, so the repair isn't the right fit as it stands. Before anything else, ask whether all the rot has really been removed. Rot often extends well beyond what's visible on the surface, and a repair that looks shallow can turn out to be deeper once you probe it. You may want to take away more of the timber to remove all the rot. Take it back until you reach sound, dry timber, and the cavity will often end up in spec on its own. That's the best outcome, since it also gives you a stronger repair.
Only as a last resort, if all the rot is gone, the remaining timber is completely sound and there's no other option, you could consider carefully removing a little more sound timber to reach the minimum depth. Removing healthy timber isn't something we'd do lightly, so if you're unsure, call us first.
Then ask how long you need to work. If you can apply it and be done in minutes, the 1 Hour is fine. If you're building and shaping, use the 4 Hour.
Then check the weather. Over 25°C, use the 4 Hour.
8. Epoxy not curing? Work through this checklist
Work through these in order:
Thickness: was it under 5 mm?
Ratio: if self-mixing, was it 3:1 by weight or volume, with clean tools for each part? If using a cartridge, did both sides dispense evenly?
Mixing: was it folded until the green tint had completely faded and it was one perfectly uniform colour?
Conditions: was it cold, damp, or exposed to rain or dew?
Product choice: was a 4 Hour used on a thin fill, or a 1 Hour on a hot day or a large mass?
If it's hard underneath and tacky on top, moving it somewhere warm and dry may help. If it's soft all the way through, scrape it back to sound timber, clean with acetone or methylated spirits, let it dry, and re-apply a fresh, correctly measured batch. Wear gloves and eye protection, since the hardener is corrosive.
Still stuck? Call (02) 7228 0625 or email info@evermend.com.au with a photo and any other detail that will help. We will do our best to help.
Frequently asked questions
How long does epoxy take to cure?
At 20°C, Evermend 1 Hour cures in 50-60 minutes and Evermend 4 Hour in 4 hours. Both are ready to sand and paint at those times. Those figures assume a properly measured and mixed batch at a suitable thickness. In colder conditions, or with a thin repair, allow longer.
Why is my epoxy still tacky?
The usual causes are an off-ratio mix, incomplete mixing, cold or damp conditions, or a repair thinner than the 5 mm minimum. Work through the checklist in section 8 to find which one applies.
Can I fix epoxy that hasn't cured?
Yes. If it's soft all the way through, scrape it back to sound timber, clean the area with acetone or methylated spirits, let it dry, and re-apply a fresh, correctly measured batch. If it's hard underneath but tacky on top, moving the repair somewhere warm and dry may help.
Does thicker epoxy really cure faster?
Often, yes. Epoxy doesn't dry, it reacts, and the reaction gives off heat. A thick repair holds on to that heat and speeds itself up. A thin one loses it to the air and the timber, so it cures at whatever the air temperature happens to be.
Can I use epoxy in a thin layer?
Evermend 1 Hour and 4 Hour are designed for applications of 5-15 mm and 5-20 mm respectively. Below 5 mm there isn't enough epoxy to hold its own heat, so the cure depends on the weather. Before doing anything else, check that all the rot has been removed, as it often runs deeper than it looks. If you're unsure, call us.
Will cold weather stop my epoxy from curing?
Both products can be applied from 0°C, but the 1 Hour and 4 Hour times on the label are for 20°C. The colder it gets, the longer the cure takes, and thin repairs suffer most because they have no self-heating to help. Damp, dew and rain can also stop it curing properly.
What is the mix ratio for Evermend 4 Hour?
Pots are mixed at 3 parts base to 1 part hardener, by weight or volume, with clean tools for each part. Mix until the green tint has disappeared and the colour is the same all the way through. Cartridges dispense the ratio for you, but check that both sides flow evenly before you start.
Should I use the 1 Hour or the 4 Hour?
Choose the 1 Hour for small repairs, spot repairs, splits and splicing, where you want to sand and paint within the hour. Choose the 4 Hour for larger repairs and projects, where you need time to build up and shape the repair, or when it's a warm day. Above 25°C, use the 4 Hour.
Can I speed up the cure with heat?
Moving a repair somewhere warm and dry can help a slow cure, provided the mix was right. Heat can't fix a wrongly measured mix, because the molecules without a partner have nothing to react with, however warm it is.
interesting stuff!