Carbon fibre/epoxy gurus help please

Hey team.

Last year I made my own carbon fibre stock. Was a really rewarding process and I learned a hell of a lot. The rifle has been used multiple times since I made it without issue. On my last outing a couple of months ago I was met with some very adverse weather in the form of torrential horizontal rain. I would have been out in it for about 4 hours. All my other hunts with the rifle have been in good weather.

I noticed at the time that there were bluish white marks forming in places on the stock that did not rub off. I can only assume that moisture managed to get into it?? After the hunt I made sure to dry the rifle really well. I took the rifle out today just to have a look and have found many marks and blemishes all over it, these are in the places where the white mares were.

Can anyone tell me what has happened? I am about to embark on a second stock build, and if I have made a mistake somewhere I would like to rectify it before I begin.

The stock is built on a powlonia blank. I laid one layer of epoxy down. When it was tacky I laid the carbon, followed by three layers of epoxy, sanding each then finishing the final coat with very fine paper. I waited until the epoxy was hard before sanding and then adding the next layer. And yea I am aware I was a touch vigorous with my initial sanding and went through the carbon in places.

My current plan is to sand the marks out, then put a coat or two on at the same time as my new stock.

I am not sure if it user error that has caused the blemishes or not?

Appreciate your thoughts.

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Hi Delphus, I’ve not seen that before. Hunt in all weather with mine for over 3 years now. Only thing I can take a stab at could be the resin not mixed fully, or the ratios not quite right. Fully cured resin is impermeable and super hard wearing. Is it tackey or gooey at all?

Yeah I thought it was weird as well. I’m thought I mixed properly and got proportions correct, but I could have gotten it wrong. Not soft at all, still hard. Odd

Yeah, just rough sand and patch over it and re finish. I do all mine in layers now. Gives a free layers in stress areas and tight curves and is easier to get a better finish. I gladwrap and tape after each application

Watching with interest to see if any epoxy gurus can shed light on possible causes. Have done 2 Paulownia CF epoxy stocks myself. Have not encountered this issue but have not had stocks out in torrential rain either. I imagine a good sand & couple of epoxy coats would remedy it. What brand of epoxy were you using ?

hasn't been by a camp fire has it ?

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Originally Posted by HILLBILLYHUNTERS

hasn't been by a camp fire has it ?

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Nope not near any open flame. After I got back from the trip I angles the heat pump to blow right o Tobit for a few hours. Was warm when I put it away, but still looked fine.

Has it been exposed to elevated temp like in the ca in summer or even outside under the sun? standard epoxy have a transition glass temperature arround 60-70deg which is the temp at which the epoxy get soft. I had hydrofoil wrapping and surface defect appearing while stored in the car in summer. Check the datasheet of the epoxy you used and look what’s the TG (transition glass temp). If you do a tapping test with a 6mm bolt (just tape it slightly, don’t hammer it) on the laminate does it sound different than on the carbon that is undamaged?

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Originally Posted by Scott Cowan

Yeah, just rough sand and patch over it and re finish. I do all mine in layers now. Gives a free layers in stress areas and tight curves and is easier to get a better finish. I gladwrap and tape after each application

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Gladwrap and tape after each epoxy layer? At what point in the drying do you wrap it?
I’ll sand and patch this one when I do the new one. It’s still a ways away as I have only just inletted it. Bedding next then I the shaping and sanding of the stock :)

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Originally Posted by tibo

Has it been exposed to elevated temp like in the ca in summer or even outside under the sun? standard epoxy have a transition glass temperature arround 60-70deg which is the temp at which the epoxy get soft. I had hydrofoil wrapping and surface defect appearing while stored in the car in summer. Check the datasheet of the epoxy you used and look what’s the TG (transition glass temp). If you do a tapping test with a 6mm bolt (just tape it slightly, don’t hammer it) on the laminate does it sound different than on the carbon that is undamaged?

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Hmm I’ll have a try tomorrow and see how it sounds. The o lay time this has been in the sun is when I have carried it. It’s my wallaby gun and I don’t spend much time sitting and glassing, just walking and shooting :)

It has only been in the safe or transported to a block in the car. Never left in there. It does look like it has heated up though

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Originally Posted by chainsaw

Watching with interest to see if any epoxy gurus can shed light on possible causes. Have done 2 Paulownia CF epoxy stocks myself. Have not encountered this issue but have not had stocks out in torrential rain either. I imagine a good sand & couple of epoxy coats would remedy it. What brand of epoxy were you using ?

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I used this kit. I spoke to the guy and he said I should use the UV stabilised epoxy co soldering it would be used in the great outdoors

https://nzfg.co.nz/product/carbon-fibre-kit-2/

It could be "blush" - some epoxies are prone to this . . . Basically not cleaning enought between coats

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Originally Posted by Tentman

It could be "blush" - some epoxies are prone to this . . . Basically not cleaning enought between coats

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Amine blush would happen straight away no? This kit you used is all good, I used it heaps. UV stabilise will help with the yellowing but it will still go bad if it goes past TG. Carbon heat really fast in summer, on a long walk with the rifle on your back it could happen. West system 105 Tg is 60deg for example Nz fiberglass kit would be similar.

About UV, it could get to this point after long exposure but you would see it goes yellowish first

release agent?

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Originally Posted by Gkp

release agent?

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Na shouldn’t be. I didn’t start the epoxy until well after the rifle had been bedded. I have it a good clean first too

I was making topwater stick baits for a while (Epoxy over cedar). And have had many go wrong and a couple go alright.
99% of my problems always came from not mixing properly and not letting the resin cure in the right temp & humidity.

Mixing rules I followed:
Use syringes to measure (never kitchen scales)
Mix in ‘pot A’ for 2minutes, pour epoxy into ‘pot B’ and mix again for 2mins.

Curing conditions need to be dry and warm.

I’ve had the milky thing happen when rain has hit an un-cured project. But not sure if this is the same as your problem.

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We see whitening of carbon finish kayaks...my understanding is that its water getting under the UV stable clearcoat. You won't see it with a pigmented paint job.
My stab in the dark is that you let an epoxy layer get to dry and because of the amine blush mentioned earlier it couldn't fully bond with the previous layer. As a result water has penetrated between the layers and has now dried out. It will be interesting to see if those blemishes peel off easily with sanding.

So I finally got round to looking at this. I started to sand it with dry paper and it quickly got gunked up. The deformed patches were actually really soft. So much so I could scrape it off with my fingernail! I changed to a knife and scraped as much as I could off, then cleaned up with some wet paper. I think I have most of it off now. I have put it in the hot water cupboard to dry overnight, then I’ll give it some more epoxy layers. I’m guessing I must have either gotten the mix wrong, or not let it cure long enough? It’s weird though as when I finished it the first time it all seemed hard.
It’s good though, a couple more layers of epoxy will be good to fix some of previous imperfections.
The main one being I originally made the LOP too short, so as you can see I have lengthened it to the correct LOP. I’ll paint it black before I epoxy and that should blend in nicely :)

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It was easy to find and fix the soft patches. I just ran the whole thing under the hose and watched the whiteness appear. It was actually quite widespread, but particularly on the butstock.