Bubbles in last epoxy/fairing layer

Got these bubbles in one area of the cured fairing/epoxy compound. It's not a structural layer.

I had bubble problems at the same stage with one board I built. The likely problem was rapid temperature changes - I built the board in a shed that got full sun in the afternoon. I built the next board in a garage where the temperatures were constant, and did not see a single bubble.
Also, I started using one more epoxy layer after the seal coat. I think Greenlight surf supply calls it "gloss coat" in their instructions. No filler in this layer, and I used a squeegee, which means the weight gain from this layer is minimal. It should still seal any unnoticed holes, or areas where the sanding was a bit too deep. If you use paint and primer (I don't, just color the epoxy), that may work in a similar fashion, but I trust the cohesion of epoxy on epoxy more, having seen plenty of paint come off over the years from many different boards and repairs.

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boardsurfr said..
I had bubble problems at the same stage with one board I built. The likely problem was rapid temperature changes - I built the board in a shed that got full sun in the afternoon. I built the next board in a garage where the temperatures were constant, and did not see a single bubble.
Also, I started using one more epoxy layer after the seal coat. I think Greenlight surf supply calls it "gloss coat" in their instructions. No filler in this layer, and I used a squeegee, which means the weight gain from this layer is minimal. It should still seal any unnoticed holes, or areas where the sanding was a bit too deep. If you use paint and primer (I don't, just color the epoxy), that may work in a similar fashion, but I trust the cohesion of epoxy on epoxy more, having seen plenty of paint come off over the years from many different boards and repairs.

Makes sense cause I put that area on around 9am and it was 30 deg by lunch

I will squeegee some more epoxy in those holes

are you sure the bung is working? I has been known for some to be blocked, not drilled out in construction.

If possible, do your repair later in the day so that the board is cooling rather than heating. I read this somewhere from someone much smarter than me.

If the glass bubbled you will know real quick when u sand it....

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Paducah said..
If possible, do your repair later in the day so that the board is cooling rather than heating. I read this somewhere from someone much smarter than me.

This seems to make sense. But I always did that when I had bubble problems, and it did not work for me at all. Maybe it's because the foam has very low temperature conductance. The cooling happens from the outside, but it may take a long time before the inside is cooled off. During that time, the shell will actually tighten around the core as it shrinks when cooling, which may make the problem worse.

I found that a lot of tips, like coating when the board is cooling, are from building prone surf boards. Those are usually a lot thinner than windsurf or foil boards, so they cool off quicker. They also usually don't have the extra sandwich layer. Some things that work well for prone boards may not work for windsurf or wing boards.

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boardsurfr said..

decrepit said..
are you sure the bung is working? I has been known for some to be blocked, not drilled out in construction.

If the foam wants to expand due to temperature differences, it will be under pressure a long time, even if the bung is working. The pressure equalization over the board does not happen instantly - rather, the time frame seems to be in the range of several hours.

On my most recent problem board that had soaked up a liter of water, the valve was working, and it had rather large openings at the foot strap inserts where a lot of water came out. Nevertheless, there were also multiple spots on the rail where water bubbled out. Those were just tiny cracks in the paint layer, much smaller than anything I would usually repair. If the inside is under pressure, the air will will find all possible ways to get out - and a thin layer of wet epoxy over a pin hole in the fiberglass layer offers very little resistance. Using slow hardener can make things worse, but I usually have to use it since I'm slow.

Side question: I thought we had to use extra slow hardener close to the foam core to avoid melting it. I only use slow after the glass layer.

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thedoor said..
Side question: I thought we had to use extra slow hardener close to the foam core to avoid melting it. I only use slow after the glass layer.

Depends on what you're doing. Heat from epoxy polymerization can become a problem if layers get thick. If you're just glassing over, no need to use even slow hardener. If you're trying to fill a big void in one go with epoxy, you can get foam melting even with extra slow hardener. I learned that the hard way. Having foam all around the void to contain the heat really does not help here.

With any hardener, if you have a lot of epoxy left over in the container you stirred it in, it can get hot enough to melt plastic. That just happens faster the faster the hardener is. "A lot" here means 1/2 cm deep or deeper. Temperature plays a big role, too. Reference for West Systems is 77 F / 25 C. A slow hardener will become fast if you're using it at 87 F, and very slow at 67 F.

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thedoor said..
I got zero bubbles when I released the vent screw so maybe Decrepit is onto something LOL

Well, Mike (Decrepit) has probably at least 100-fold more experience building and repairing boards than I do, so that's quite likely.

My problems when building a board happened when following the "standard" instructions, where the release valve is put in last. So when I got bubbles in the seal coat, there was no other way out (the top had a sandwich layer which prevented air escape).
The recent problem board where water was coming out at multiple spots had a Goretex valve. It's possible that these "automatic" vents let less air through than regular vents.

If you're curious, you could tape the holes in the new repair, close the vent screw, and let the board sit in the sun for a couple of hours to warm up really good. Then remove the tape, open the vent screw, and see if you now get bubbles, with more pressure in the board. But you'd have to be really curious and willing to sacrifice the board, since heating the board up like that may cause delamination.

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lemat said..
After 30 years of building eps core boards i learn that you should seal shaped foam with epoxy micro slurry. Il like to add a touch of silica and color in the mix to spread it thoroughly everywhere. Spend a little time then every step after is easier and effective particulary waterproofing.

Ok cool. How exactly do you make the slurry? Some silica plus epoxy?

So much crap talking in this thread.

You can do all this and still have air wanting to escape the EPS if the temp (ie: inside pressure) goes up.
There is no such thing as as a windsurf board with a perfectly sealed core that WON'T push out some air - and even more so through a big ding.
If you laminate over that ding, escaping air will push the glass away.
Then, when u mix a filler, there may be air bubbles in the filler just from stirring it and they will rise. You can pop them all and overwork the filler and get rid of them..... and then still have some air coming out of the board.

It is not the case that this is due to a poor board build and sealing the core will prevent it. Its not impossible for air exit to occur on a sandwich board. Once there is a hole in it, it may as well be a block of EPS again yeah? There is no sandwich in that area now.
Blowtorch over filler might fix the bubbles, but air will still come out over the next few hours.

The only sure approach is falling temp. Even then, sometimes it needs a second fill for some bubbles. If you want perfect repairs you will fill and sand at least twice before priming anyway.

My big tip in addition to temp, is ditch the boat repair products if you are going to do more than very infrequent repairs.

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Mark _australia said..
So much crap talking in this thread.

You can do all this and still have air wanting to escape the EPS if the temp (ie: inside pressure) goes up.
There is no such thing as as a windsurf board with a perfectly sealed core that WON'T push out some air - and even more so through a big ding.
If you laminate over that ding, escaping air will push the glass away.
Then, when u mix a filler, there may be air bubbles in the filler just from stirring it and they will rise. You can pop them all and overwork the filler and get rid of them..... and then still have some air coming out of the board.

It is not the case that this is due to a poor board build and sealing the core will prevent it. Its not impossible for air exit to occur on a sandwich board. Once there is a hole in it, it may as well be a block of EPS again yeah? There is no sandwich in that area now.
Blowtorch over filler might fix the bubbles, but air will still come out over the next few hours.

The only sure approach is falling temp. Even then, sometimes it needs a second fill for some bubbles. If you want perfect repairs you will fill and sand at least twice before priming anyway.

My big tip in addition to temp, is ditch the boat repair products if you are going to do more than very infrequent repairs.

Thanks Mark. It's certainly more water tight there than it used to be.

FYI no PVC sandwhich in windsurfer LTs I guess that is how they can sell them such low prices

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Mark _australia said..
My big tip in addition to temp, is ditch the boat repair products if you are going to do more than very infrequent repairs.

Hey Mark, what's the problem with boat repair products? What would you recommend?. I am not trying the derail the thread, I'm curious.

Thanks. Nope, USA. Do you know any brands sold here?

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Mark _australia said..
My big tip in addition to temp, is ditch the boat repair products if you are going to do more than very infrequent repairs.

Maybe that's a good tip, maybe not. But regardless, the type of epoxy used has nothing to do with the original problem. Using an epoxy with a faster set time might have reduced the number and/or size of the bubbles, but the OP could likely have used the fast hardener.

Talking trash about all "boat repair products" is, at best, very questionable. Perhaps it is based on some negative experience in the past - but that's experience in Australia, not the US. Even if Mark had bad experiences with the same epoxy (West Systems I assume), this brand is manufactured by a different company in Australia and New Zealand.

As for the other arguments against "boat repair products", they are either irrelevant or actually wrong:
1. UV sensitive: The West Systems manual states that the epoxy is UV sensitive, but the actual effect that US has is a discoloration (yellowing) over time. In a repair that will be painted, the epoxy if protected from the UV by the paint layers, and even if the epoxy would yellow over time, it would not be visible.
2. Measuring: West Systems is by far the easiest to accurately measure if you use the West System pumps. I have plenty of scales, but use the pumps not just for repairs, where they are great, but also when building boards. Never had any issues with poor polymerization.
3. Lower tensile and compressive strength: Resin Research data sheets list a compression yield of 13,800 psi and a tensile strength of 10,500 psi. West Marine lists compression strength of 11,000-12,000 psi and a tensile strength between 7,000 and 8,000 psi, depending on the hardener used (sources: www.westsystem.com/compare-epoxy-physical-properties/ and resinresearch.net/wp-content/uploads/2023/06/surf-pro-TDS_V4.1.pdf). So yes, West Marine is lower, but the difference is irrelevant. even the lowest spec West Marine epoxy has a tensile strength of 3 tons per square inch.
If you end up needing to make a repair on vacation and just buy some epoxy at the hardware store or supermarket, you usually get stuff spec'd to only 3,000 or 5,000 psi. I've had to use this stuff a few times, too, and those repairs (always with glass) so far have held up over the years.
I started using West Marine epoxy for repairs more than 10 years ago based on recommendations on the iWindsurf forum by others with experience. The only problems I ever encountered were due to my inexperience, not due to the epoxy. I have had one repair at a high load area (at the rear foot) on my go-to board that has lasted many years without problems. This repair was at the edge of the underlying wood re-enforcement, where the original board construction failed simply because I often put my foot a bit outside of the strap, covering the edge with my heel. The repair (with plenty of pour foam and vacuum sandwich reconstruction with West Marine epoxy) has held up without problems for 7 or 8 years.