Hardening epoxy and curing in poor weather

Hi there everyone,

New here, just wondering what the best combination of glass, carbon and fillers provide the hardest epoxy without becoming brittle? Also any tips on glassing in 15°C and high humidity? Had a few failures recently even though curing by radiator with temp set at 26°C

Not sure what the application is here, but epoxy by nature is not as brittle as polyester resin. Chopped fiberglass (just cutting up into small pieces with scissors works for small amounts, like a fin gusset / fairing) makes a good thickener & reinforcement. Microballoons works as a thickening agent, but doesn’t add the strength that chopped fiberglass does.

15 degrees Celsius is not warm enough for working with epoxy resin, 26 degrees should be. But the setting on the radiator is not necessarily the temperature in the room. You can warm the epoxy before use by placing in a tub of warm water, some use a microwave oven but I have never tried it. Placing a small tub or cup of resin in warm water works for me. I have also, in a pinch, used a heat gun to accelerate the applied resin/hardener in cold damp conditions.

If you are not familiar with epoxies I want to point out that there is a big difference between pot life, kick time and curing time.

With the resins I use 15 deg Celsius should be ok, even if it’s too low or on the lowest end according to the manufacturers manual.
But it’s quite normal that with these low temperatures the full cure can take way longer than expected. Im not talking hours , but a week and longer. Additional heat after the initial cure is good, and if you have the opportunity to get the temperatures even higher go for it.
If the resin gums up when you sand it’s not fully cured and you should stay away for health reasons.

The industry standard for composites is a tempering process where you post cure your parts in an oven after the lamination and the temperatures are more like 80 deg Celsius over several hours -depending on the application.
This baking process helps to get a very good matrix that lives up to the properties that carbon needs to show it’s potential.

I’m just saying this cause it’s a ( sad) truth that it is just not possible to get optimum results in a garage setup especially on a rainy day- no matter what surfacing agent you might find.

There are resin systems that are more suited to less than optimal conditions though that should do the job.


dehumidifier and a heater.

Heater without combustion.
Flame powered heaters produce CO2 that combines with moisture forms “amine blush.”

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There is a Jimmy Lewis technique that would help prevent blush.
I will try to dig up the link to that video.

This method prevents moisture/humidity and CO2 from reacting with curing epoxy resin (blush reaction). Starting at 5:25 in this video.

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