Rough surface on epoxy coating

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opti_builder
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Rough surface on epoxy coating

Post by opti_builder »

Hello,

I recently purchased the Optimist kit and started work on it. I ran into some trouble doing the first coating of the plywood with epoxy (just the daggerboard trunk pieces so far). I'm wondering if anyone can give some suggestions to help.

After the epoxy finished curing, the finished surface is extremely rough. It looks like the roughness may be caused by small air bubbles trapped in the epoxy. How can I keep this from happening in the future? Let me add some additional information that may be relevant:
- The epoxy is the marine-grade stuff sold by BoatBuilder Central, with the medium speed hardener.
- It seemed like there were some air bubbles in the resin-hardener mixture before applying, but it's hard to imagine this much roughness resulted from the number bubbles I saw. (I stirred the resin/hardener mixture pretty vigorously based on what was shown in the YouTube clip, but from what I have seen in other places, mixing more slowly may help prevent bubbles?)
- The temperature in the garage where we applied it was about 58 degrees (probably on the low end of the recommended range for the medium hardener)
- We applied it with a brush.

fallguy1000
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Re: Rough surface on epoxy coating

Post by fallguy1000 »

It is the temps. Outgassing is more common in cooler temps. Epoxy can flow a lot better when it is applied about 75F.

I normally won't apply epoxy below 60f.
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VT_Jeff
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Re: Rough surface on epoxy coating

Post by VT_Jeff »

If it's due to outgassing, subsequent coats should not have the same issue. Add a skim coat and see if its smoother.

Btw , best to recoat wet on wet, or at least before the first coat is fully cured, to eliminate the need to sand between coats.
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narfi
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Re: Rough surface on epoxy coating

Post by narfi »

I had some REALLY bad surfaces from this when trying to heat an area I had just epoxied. Since then I have figured out the 'science' behind it and have (so far) not had the same issues.

It is logical once you think about it, but I never thought of it before doing this.

When air heats up, it expands, when it cools down it contracts.
There is air in wood and plywood.
Therefor when wood heats up, it releases expanded air, and when it cools down it sucks air in.

The way we can use that to our advantage is to always apply epoxy at the beginning of the cooling cycle (obviously still operating in the temp ranges recommended for the epoxy)

There are a couple of ways to do this,
You could heat the wood up before you start, and then once the epoxy is applied let it cool down to room temperature.
For me working outside in a tent it is easy though, I apply my epoxy in the evening after the sun has stopped heating the tent and it is starting to cool down.

What this does is instead of blowing bubbles in your epoxy and making the rough surface you found, it sucks the epoxy in slightly and gives it a better bond and better protection.

For your rough surface, just sand it smooth and give it another coat, no worries :)
The awesome thing about working with this stuff is it is nearly impossible to screw up worse than you can just add some epoxy and sand off the extra and come up with what you wanted in the first place :)
I think my fs17 might have ended up just being a 17x7ft block of thickened epoxy that I sanded a boat shape out of :P

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VT_Jeff
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Re: Rough surface on epoxy coating

Post by VT_Jeff »

narfi wrote: Thu Apr 15, 2021 11:32 am I had some REALLY bad surfaces from this when trying to heat an area I had just epoxied. Since then I have figured out the 'science' behind it and have (so far) not had the same issues.

It is logical once you think about it, but I never thought of it before doing this.

When air heats up, it expands, when it cools down it contracts.
There is air in wood and plywood.
Therefor when wood heats up, it releases expanded air, and when it cools down it sucks air in.

The way we can use that to our advantage is to always apply epoxy at the beginning of the cooling cycle (obviously still operating in the temp ranges recommended for the epoxy)

There are a couple of ways to do this,
You could heat the wood up before you start, and then once the epoxy is applied let it cool down to room temperature.
For me working outside in a tent it is easy though, I apply my epoxy in the evening after the sun has stopped heating the tent and it is starting to cool down.

What this does is instead of blowing bubbles in your epoxy and making the rough surface you found, it sucks the epoxy in slightly and gives it a better bond and better protection.

For your rough surface, just sand it smooth and give it another coat, no worries :)
The awesome thing about working with this stuff is it is nearly impossible to screw up worse than you can just add some epoxy and sand off the extra and come up with what you wanted in the first place :)
I think my fs17 might have ended up just being a 17x7ft block of thickened epoxy that I sanded a boat shape out of :P

100% agreed that that is the best way to go Narfi, if it's feasible. My shop is basically a constant temperature, 55 degrees. I used to warm it up before working in it but I don't bother anymore, I just wear more clothes. I do warm up my expoxy in a crock pot. Applying warm epoxy to cold plywood will logically lead to outgassing, which I don't really worry about, because I'm always adding another coat on anything I care about being smooth anyway. As I mentioned above, the second coat should not suffer from outgassing since the plywood is already sealed by the first coat.

I also agree with your assessment that it's not too critical anyway, just sand it smooth. Also, any critical surfaces should probably be glassed, which is going to leave you with a weave to fill, which when filled should not outgas because see #1.

"I think my fs17 might have ended up just being a 17x7ft block of thickened epoxy that I sanded a boat shape out of :P"

My thoughts exactly!
There are only two seasons in Vermont: boating season, and boat-building season.

Completed Paul Butler 14' Clark Fork Drifter
Completed Jacques Mertens FS14LS + 10%, Build Thread
Started Iain Oughtred Tammie Norrie

opti_builder
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Re: Rough surface on epoxy coating

Post by opti_builder »

Thanks all- appreciate the suggestions!

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