Why epoxy does not cure?

There are a lot of different kinds of epoxy, and the uses for these materials can be very different. The most common questions that I get when people find out that we make epoxy are, “What is epoxy?” and “What’s the difference between resin and epoxy?” I’d like to address both of those questions in this blog post.

Some Definitions:

Epoxy is a generic term used to denote any material containing an epoxide group. It is a functional group made up of an oxygen atom covalently bonded to two carbon atoms in such a way as to create three rings (the oxygen atom becomes part of both the benzene ring, with C-C double bonds, and the ether ring).

What to do if epoxy does not cure?

If you notice your epoxy is not set, it may be caused by one of several reasons. Taking these steps to diagnose the problem will likely fix the issue:

  • Check the resin to hardener ratio
  • Check if the resin and hardener are expired
  • Check the temperature
  • Check the surface
  • Check the mixing
  • Check the curing time
  • Check the curing process

Why does epoxy take so long to cure?

You may have noticed that your epoxy starts out in a liquid state and gradually thickens as it cures. In order for the curing process to begin, the resin and hardener must mix together.

The components are typically blended with a mechanical mixer at high speed, which thoroughly blends the two materials and begins the curing process.

However, it takes time for the resin to travel from the mixing cup into other parts of the epoxy system.

After all of the material is mixed together, then comes a chemical reaction, which also takes time to complete.

The process of epoxy curing is exothermic, meaning heat is released during this reaction. The heat produced during cure can slow down or stop some reactions from occurring at lower temperatures.

Since many resins and hardeners are temperature sensitive, an ambient temperature that is too cold (50 degrees Fahrenheit or less) can delay or halt chemical reactions and extend your cure times indefinitely

Why is my resin still soft?

There are many reasons why your epoxy may not cure. It is important to identify the root cause so that you can fix the problem and prevent it from happening again.

If you’re wondering, “Why is my resin still soft?” some of the most common causes include:

  • Not using adequate UV light with a UV resin
  • Using too thick a layer of resin

These are just two examples; there are other reasons that epoxy does not cure. It’s important to identify why your epoxy did not cure so that you can avoid this mistake in the future.

Why is my resin bendy after a week?

If you have ever had a resin piece that was nice and hard at first but got bendy after a week, there is a good chance it was due to UV light exposure.

While it might seem surprising, some resins are very sensitive to UV light. These types of resins will cure faster when exposed to sunlight than compared to curing in the dark.

On the other hand, some resins are not UV resistant and will be softer if they are left in the sun for an extended period of time.

If you are using one of these non-UV resistant resins (such as Alumilite Amazing Casting Resin), then you need to protect your pieces from sunlight for at least 72 hours after casting them.

If your piece has been cured for more than 72 hours and still gets soft after being in the sun for a while, then your resin might be sensitive to UV light!

Why did my resin not harden?

There are several things that could cause your resin not to harden. Sometimes, the resin and hardener were just not mixed properly, or the mixture is too thin.

The epoxy could also be too old or improperly stored. If, for example, it was stored in a hot garage during summer, it could have lost its potency even if it wasn’t opened yet.

Finally, it may also be contaminated; you don’t want to store your materials in a dirty area where bugs can get into them and ruin them!

To prevent this from happening again:

How do you know when epoxy is fully cured?

“How long does epoxy cure?” is probably the most common question asked of any seasoned craftsperson. The truth is, it depends on numerous factors. But before we go there, let’s talk a bit about what “curing” means.

Curing epoxy is basically—wait for it—a process by which the epoxy becomes hard enough to stick to stuff. The longer you leave it sitting around outside of your project before applying pressure to see if it’ll hold, the more likely it is to fail when you do so; the less time you leave it on top of your project before pressing down and seeing how much pressure your glue gun can handle, the less likely it will fail under load; and so on.

But that’s not all—there are other factors involved as well. Epoxy will begin curing when its volume decreases while still maintaining its liquid state (think: thick syrup), but some types cure faster than others (think: wet paint).

Humidity makes a difference as well; high humidity causes curing times to increase dramatically while low humidity causes them to decrease dramatically (think: paint drying).

While temperature can have an effect on each individual batch, generally speaking, there seems to be little correlation between curing and temperature beyond those variables we’ve already discussed (think: hot glue works best at low temperatures while cold glue works best at high temperatures). So don’t fret if one day your epoxy isn’t cured yet.

Does temperature affect epoxy curing?

Yes. In fact, temperature affects the working and curing time of the resin and hardener.

The higher the temperature, the faster the working time is reduced, while the curing time increases. For example, at 70 degrees Fahrenheit (21 degrees Celsius), a typical epoxy formulation has a working time of approximately 15 minutes and will cure in 10 hours.

If you raise the temperature to 90 degrees Fahrenheit (32 degrees Celsius), that same formulation will have a working time of 8 to 9 minutes and a cure time of 7 to 8 hours.

On the other hand, if you lower your room’s temperature from 70 to 50 degrees Fahrenheit (10 degrees Celsius), your 15-minute working time extends to 22 minutes, while your 10-hour cure time stretches out to 14 hours.

How long does 5 Minute epoxy take to cure?

  • As a general rule of thumb, allow 24 hours per inch of thickness for a full cure and two weeks before heavy use (e.g., your new surfboard should lie on its back for two weeks after you put your last layer of resin on it.

This will vary based on factors that slow down or speed up the chemical reaction between resin and hardener: temperature, humidity, amount of sunlight, amount of air exposure during application, and amount of air bubbles you introduce when mixing.


To sum up, I’ve tried to give you some insight into what might have gone wrong in one of your resin projects. If you followed the instructions carefully and took note of any variables (like temperature), then a more thorough study of your work may be in order.

The best way to learn is by doing, so go out and make mistakes now! And remember: epoxy resin can be a hard-wearing and long-lasting material, but it’s important to understand why it doesn’t cure properly.

There are many factors that can influence how long it takes for resin to cure, from temperature to the amount of catalyst used.

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Martin Flood

Martin Flood has been working in the construction industry for over 20 years as a general contractor with expertise in remodeling projects that are large or small. He has furthered his career by specializing in epoxy resin flooring, providing excellent service to both commercial and residential clients. Martin’s experience enables him to offer professional advice on how to choose the right type of project based on your needs and budget.

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