View Full Version : Adding Epoxy Thinner: How much is too much?

05-20-2018, 07:09 PM
I have a hairline crack that I want to fill with epoxy.

Not a religious issue because I will be bonding FG tape over and around it, but it seems like a nice-to-have if I can fill it.

Have used thinned expoxy in the past, but not that thinned.

By percent weight, what is the maximum amount of thinner that I can add to a small batch of epoxy to maximize it's penetration, but not wind up with a mess that never cures?

Mike Seibert
05-20-2018, 07:27 PM
The West guys insist that you should not thin their products. I am not epoxy scientist like those guys are and am enough of a rookie that I would never ignore something like that.

There are much thinner epoxies than West. I recently used some MAS low viscosity that was much thinner but I don't think they recommend that for structural bonding. But at least it may fill the void.

My guess is that someone with lots more experience will have a good idea.


Ian McColgin
05-20-2018, 07:42 PM
WEST is thin enough. Just use a syringe to really inject it into the crack. Move carefully from one end making sure that you are flooding ahead of the syringe to ensure that you're not entrapping air.

05-20-2018, 07:45 PM
What's the material and use of the cracked part?

Todd Bradshaw
05-20-2018, 08:00 PM
A "hairline crack" which will later be covered with fiberglass tape shouldn't need any filling at all, or need any further strengthening before taping. If the spot and crack are structural, open it up enough to fill the area with a real epoxy/filler mixture, rather than trying to get resin to soak down into a crack. Diluted epoxy always has questionable strength and questionable sealing power. If you are trying to fix something with epoxy resin, use 100% solids resin.

05-20-2018, 08:03 PM
“V” the crack and warm the resin to make it less viscous

wizbang 13
05-20-2018, 08:19 PM
Use the slowest hardener ...the longer it is wet, the longer it will creep down. That don't require a degree to figure out.. common sense.
I never thin it.That's daft.
Warming makes it more watery, but it also speeds up the cure time, bit of a dilemma.

05-21-2018, 12:07 AM
Warm the part, not the epoxy.

05-21-2018, 01:10 AM
Woops forgot to add that About warming resin, obviously it’ll make it kick a bit sooner but if you’re quick should be fine. I do it all the time, got a warming globe under the container for winter. Warming the part will also warm the epoxy once its on there. Another thing you might try is waving a heat gun over it, but be very gentle, to encourage the goop down into crack.
Can you flex the crack open? I tried to think of another way to put that!!!

05-21-2018, 04:32 AM
I would use a hair dryer, not a heat gun. You want to warm it, not fry it. In building sailboat models I routinely warmed both the deck and the epoxy with a hair dryer to get the epoxy to penetrate. Works fine on smallish areas. Use the slowest hardner.