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Old 19-06-2024, 05:24   #1
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Uncured expoxy filler - advice needed

Hi all,

24 hours ago, I mixed West Systems epoxy and added filler, and filled in some old bolt holes on the deck. For the first time in two years, after many gallons of epoxy, I can say I'm not sure 100% that I got the ratio right.

Today:

The filler has not cured entirely. I can push my fingernail and make a mark. It's not sticky or tacky, it just hasn't fully hardened - it's too soft.

I'm in the Caribbean, and due to the temps here epoxy cures *very* fast. 24 hours is always enough time to harden and sand - but not this time.

My main question is - is it likely to fully harden, slower than normal, in the coming days, even if the ratio was wrong?

Or, will it never harden fully?

I'm trying to decide whether to remove everything (13 bolt holes worth), and redo, or just wait....

(another thing - the pot I used, the leftovers have a "flexibility" to them. Nothing is sticky/runny - just more flexible than normal.)

Cheers for any advice!
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Old 19-06-2024, 05:39   #2
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Re: Uncured expixy filler - advice needed

If it has picked up some catalyst it should cure. If this is just a fill in and not structural I would wait and see.
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Old 19-06-2024, 05:50   #3
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Re: Uncured expoxy filler - advice needed

I'd remove it and start again.

An expert on epoxy resins once explained it to me when I first started with epoxy resins. He said you have to have the right ratio- "a bit like if you were organizing a dance function, you need to have just the right number of guys to gals". Too many guys or too many gals can destroy the function.

Why do I remember that from 40 years ago!
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Old 19-06-2024, 06:11   #4
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Re: Uncured expoxy filler - advice needed

I've worked with WEST epoxy for years and years and have occasionally had instances where it would not set up. Reasons were varied, old epoxy, incorrect mix ratio, etc.
Even despite leaving it to cure for days and days, it would not set up correctly. My guess is an incorrect mixture.
I've especially had problems with WEST as it comes in those one use sachets, where I did not squeeze out all of the hardener, easy to do.

Though it might be an onerous task, I'd remove it and start again. However, if you are only using the product as a filler and not structurally, give it a week or so and re-assess.
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Old 19-06-2024, 06:13   #5
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Re: Uncured expoxy filler - advice needed

I just had a similar situation in the Midwest USA this week where a short overnight 8 hours was not enough to cure 4:1 epoxy plus filler but seemed fine a warm day later and sanded good this morning 32 hours later.

When epoxy is mis-mixed, the unconsumed resin or hardener acts to plasticize or modify the final mix, which may be what you feel with the stuff in the cup. If it does not 'straighten up and fly right' within a day I would consider removing and redoing if the patching is mission-critical.

Epoxy is a different animal than pre-promoted and catalyzed (accelerated) resin system like polyester. Those can eventually go off in the can even without MEKP (catalyst/accelerant) and can be sped up or slowed down during use by slightly adjusting the percentage of MEKP. Too much MEKP is not good either. I used to have a drawing in my shop of a surfboard glasser making a polyester gloss coat mix 'too hot' with catalyst and having to run it out of the shop while it smoked.
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Old 19-06-2024, 06:15   #6
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Re: Uncured expixy filler - advice needed

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Originally Posted by Turtle Blues View Post
If it has picked up some catalyst it should cure. If this is just a fill in and not structural I would wait and see.
This is NOT true for epoxy. There is no “catalyst” in epoxy. The ratio must be right because the two parts react together in a fixed ratio. With most formulations you must be within 10% to get the proper physical properties of the cured product. Adding more of one or the other just results in a gooey mess, not a faster or slower cure. If it hasn’t cured in the normal time for that temperature, it never will.

Polyester based resins are different. The second ingredient IS a catalyst, and the amount of it does change the cure rate without affecting the end result. You can still add too much or too little, but there is a fairly wide range.
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Old 19-06-2024, 06:34   #7
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Re: Uncured expoxy filler - advice needed

Start again.

Unlike polyester or vinylester resin in which the catalyst (usually MEKP) accelerates the chemical reaction between the ester and styrene monomer to form the chemical bond, with West Systems epoxy the resin and hardener react together to form the chemical bond.

The West Systems hardener is not a catalyst. Some may think, albeit incorrectly, that the hardener is a catalyst, simply because the chemical reaction between the resin and hardener is exothermic.

If it hasn't cured, because of an incorrect mixing ration, it will never cure because there won't be enough molecules of hardener to form sufficient cross links with the molecules of epoxy.
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Old 19-06-2024, 07:00   #8
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Re: Uncured expoxy filler - advice needed

I have both the WEST system pumps and also the small one use sachets.

Using the pumps, I've never had a problem, but using the small one use sachets I have problems, invariably due to me not squeezing out all of the product, so can attest that WEST is very sensitive to correct ratio.
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Old 19-06-2024, 08:01   #9
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Re: Uncured expoxy filler - advice needed

Update:


I decided to sand the "excess" on top of each hole (I just did this 30 minutes ago).

The sanded filler seems much firmer - I'm reluctant to test properly with my nail, as the surface looks so perfectly fair at the moment. I'll wait until the Caribbean sun does it's thing today, and then test properly. I've also tested from inside the boat where filled the bolt holes exit - that side is very hard.

Believe me, I take on board all advice and thank everyone. I don't take short-cuts - if the outer surface is not fully hardened by this afternoon, I will drill and remove all of the epoxy filler and start over.

(re-doing would be one of the smaller epoxy jobs I've done on this boat - I've been through possibly 10 gallons of West Systems over two years, and never once had this issue - hence my question here)
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Old 19-06-2024, 08:16   #10
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Re: Uncured expoxy filler - advice needed

Tiny bubbles, coming from the filler you added and the exotherm, can cause that. The epoxy is cured, but not quite full strength. It would also explain why the underside is hard.
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Old 19-06-2024, 09:01   #11
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Re: Uncured expoxy filler - advice needed

I've tested the top-side, I can push the point of a screw into it, but it takes a bit of force.

Better than before, but it's looking like I'll have to redo the job.
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Old 19-06-2024, 11:53   #12
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Re: Uncured expoxy filler - advice needed

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Originally Posted by coopec43 View Post
I'd remove it and start again.

An expert on epoxy resins once explained it to me when I first started with epoxy resins. He said you have to have the right ratio- "a bit like if you were organizing a dance function, you need to have just the right number of guys to gals". Too many guys or too many gals can destroy the function.

Why do I remember that from 40 years ago!
I had an old timer give me the exact same explanation. Unlike polyester, epoxy is a molecule-to-molecule reaction. Too many molecules of either part will leave left over molecules of the other and it won't cure properly. If it's just little bit off you won't notice but too far off and you'll have problems. The hardener can also go bad over time. Was it clear or amber colored? When it starts turning brown and smells like ammonia, throw it away.
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Old 19-06-2024, 18:41   #13
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Re: Uncured expixy filler - advice needed

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Originally Posted by SailingHarmonie View Post
This is NOT true for epoxy. There is no “catalyst” in epoxy. The ratio must be right because the two parts react together in a fixed ratio. With most formulations you must be within 10% to get the proper physical properties of the cured product. Adding more of one or the other just results in a gooey mess, not a faster or slower cure. If it hasn’t cured in the normal time for that temperature, it never will.

Polyester based resins are different. The second ingredient IS a catalyst, and the amount of it does change the cure rate without affecting the end result. You can still add too much or too little, but there is a fairly wide range.

I couldn't agree with you more!
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Old 19-06-2024, 18:48   #14
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Re: Uncured expoxy filler - advice needed

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Originally Posted by stevead View Post
Start again.

Unlike polyester or vinylester resin in which the catalyst (usually MEKP) accelerates the chemical reaction between the ester and styrene monomer to form the chemical bond, with West Systems epoxy the resin and hardener react together to form the chemical bond.

The West Systems hardener is not a catalyst. Some may think, albeit incorrectly, that the hardener is a catalyst, simply because the chemical reaction between the resin and hardener is exothermic.

If it hasn't cured, because of an incorrect mixing ration, it will never cure because there won't be enough molecules of hardener to form sufficient cross links with the molecules of epoxy.
I agree with you completely. I just wish people would not give their "expert" advice when they know nothing about polyester resins or epoxy resins.
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Old 19-06-2024, 23:42   #15
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Re: Uncured expoxy filler - advice needed

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Originally Posted by coopec43 View Post
I agree with you completely. I just wish people would not give their "expert" advice when they know nothing about polyester resins or epoxy resins.
Thank god we have an expert like yourself to provide guidance and clarification
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