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Old 20-11-2020, 18:34   #1
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How do I know if they're blisters?

I've been sanding for what seems like ten or fifteen years now, and I'm pretty sure I've got some blisters. There were some high spots in the gel coat, is what I mean, so when I continued to sand there were exposed areas of the laminate.

I didn't see any obvious delamination, and I didn't want to grind unnecessarily because the hull is only about 1/4" thick.

I also didn't see any weeping, but that's probably because I didn't grind too deep. But if I did try to break any, would I still see weeping? The hill has been out of the water (except Florida rain) for at least three years, probably more.

1. What's the best way to tell if there's delam? I figure I can try to poke them with a marlin spike or something, grind them down (but how far?), or tap with a hammer to see if they sound funny.

2. Would they still weep after years out of the water?

3. Is there something else they could be?


There are two photos. I posted some of these earlier to ask about the weird kind of crazing in the gelcoat that's visible in both (which sooner said don't worry about), but the more I look up, the more it seems like they really are blisters.

Anyway sorry if this sounds like a broken record.
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Old 20-11-2020, 19:12   #2
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Re: How do I know if they're blisters?

Those are small BB size and I've had them too. They should have a little moisture in them, although it may be minimal. break a few bigger ones with a pocket knife and see what's there the next day, a brown spot or even brown drip hanging possibly. Or it may just be damp. It's my belief that when you have time, sanding and just letting those little ones air dry for a few weeks, followed by a rolled on epoxy resin layer will likely fix it long term.
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Old 20-11-2020, 20:12   #3
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Re: How do I know if they're blisters?

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Those are small BB size and I've had them too. They should have a little moisture in them, although it may be minimal. break a few bigger ones with a pocket knife and see what's there the next day, a brown spot or even brown drip hanging possibly. Or it may just be damp. It's my belief that when you have time, sanding and just letting those little ones air dry for a few weeks, followed by a rolled on epoxy resin layer will likely fix it long term.
Do you think the same about the really tiny ones in the first picture, next to the big one?
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Old 21-11-2020, 09:34   #4
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Re: How do I know if they're blisters?

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Do you think the same about the really tiny ones in the first picture, next to the big one?
A bit hard to see but yes, those are the ones I was talking about. It's just starting to show moisture with those. I think they will open up if you just sand them (?) then let dry. They are minimal, I see little cracks/crazing also? I wonder if this is due to a paint issue? Is that white paint or gel? If gel it appears to have hardened too fast and is crazed. You could sand the whole thing, let it dry for a while, fill any of the bigger blisters, then roll on epoxy and probably be good for years.
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Old 21-11-2020, 11:02   #5
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Re: How do I know if they're blisters?

It's gel. I opened up a few and they don't seem to be too deep. They're completely dry inside but tapping on the with a marlin spike I got the good old dull thud. I pushed in pretty hard with spike and heard the crunch, picked out a little laminate.

Bad news, there are a few hundred, but at least they're really small, about the size of a pencil eraser.

I was thinking of trying the boring bit to open them up. I think that's the right word for it (it's a drill bit that basically drills a cone shape). I only need to go down a millimeter or two. just trying to save some time opening all of them up.

I'm honestly not sure what all that crazing is. it looks exactly like the gel coat where there were blisters so I thought maybe it was just a whole bunch of little tiny blisters, but tapping around the area I didn't get any dull thuds. Maybe in those spots, and thankfully there aren't too many of them, where that's happening I'll just grind away a layer under the gel coat and then lay up a layer or two of 10 oz cloth with epoxy resin.
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Old 21-11-2020, 11:20   #6
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Re: How do I know if they're blisters?

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I didn't see any obvious delamination, and I didn't want to grind unnecessarily because the hull is only about 1/4" thick.

.
What type boat has a hull which is 1/4 “ thick?
Most blisters I’ve seen are deeper than that.
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Old 21-11-2020, 11:23   #7
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Re: How do I know if they're blisters?

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What type boat has a hull which is 1/4 “ thick?
Most blisters I’ve seen are deeper than that.
It's a cal20. The hull is thicker in some places, but by my best guess it's only 5 or 6 mms at the thinnest part.
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Old 21-11-2020, 11:46   #8
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Re: How do I know if they're blisters?

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It's gel. I opened up a few and they don't seem to be too deep. They're completely dry inside but tapping on the with a marlin spike I got the good old dull thud. I pushed in pretty hard with spike and heard the crunch, picked out a little laminate.

Bad news, there are a few hundred, but at least they're really small, about the size of a pencil eraser.

I was thinking of trying the boring bit to open them up. I think that's the right word for it (it's a drill bit that basically drills a cone shape). I only need to go down a millimeter or two. just trying to save some time opening all of them up.

I'm honestly not sure what all that crazing is. it looks exactly like the gel coat where there were blisters so I thought maybe it was just a whole bunch of little tiny blisters, but tapping around the area I didn't get any dull thuds. Maybe in those spots, and thankfully there aren't too many of them, where that's happening I'll just grind away a layer under the gel coat and then lay up a layer or two of 10 oz cloth with epoxy resin.
Yeah, If they seem dry I wouldn't put a ton of time into it on an old 20 footer. Put a barrier coat on it and sail! You start boring holes and filling and etc and suddenly you are in a mess!
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Old 21-11-2020, 12:08   #9
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Re: How do I know if they're blisters?

What you’ve done, and what Cheechako has said.

.
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Old 21-11-2020, 14:37   #10
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Re: How do I know if they're blisters?

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Yeah, If they seem dry I wouldn't put a ton of time into it on an old 20 footer. ...
Well, I had hoped to sail it down into the Caribbean a good distance. Maybe to puerto rico, so I wanted to try and make it nice and strong.
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Old 22-11-2020, 10:46   #11
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Re: How do I know if they're blisters?

answer you cannot. The good news is you need not care. Just finish your work and launch. Haul in a few months and you will see if you have blisters.
Blisters generally cannot be seen after the boat dries.

IMHO the small spots bothering you are likely imperfections in the hull material and not a structural defect. Cutting through all the uninformed propaganda there are zero documented cases of blister caused structural damage. If your hull really as thin as you state it is very unlikely that it is made from material vulnerable to blisters.
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Old 22-11-2020, 10:48   #12
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Re: How do I know if they're blisters?

Delaminated GRP will be soft. It is dangerous to judge from pictures only, but to me it seems gelcoat only.
Sand the blisters down to the GRP, apply new gelcoat, sand to flush and sail away.
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Old 22-11-2020, 10:52   #13
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Re: How do I know if they're blisters?

Here’s an informative article regarding blisters. Hope it helps.
http://www.yachtsurvey.com/BuyingBlisterBoat.htm
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Old 22-11-2020, 11:57   #14
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Re: How do I know if they're blisters?

I am reminded of that old Crocodile Dundee film. You know the bit " Thats not a knife. THIS is a knife..."
Well, those aren't blisters....
Seriously, sanding & epoxy coating should do the job you need. And the drill bit you mentioned is a countersinking bit. I would be very careful using that on thin fibreglass as they are surprisingly effective in sinking a hole....
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Old 22-11-2020, 15:27   #15
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Re: How do I know if they're blisters?

Thanks for the article. Most of mine are less than an inch based on the rapping test. For those that approach 1 inch, I'm going to grind them out with a dremel and patch them. That shouldn't be too extensive.

As for the sanding many of you are recommending, do you mean to sand into the laminate, or just through the gel coat, or something else?

I bought epoxy barrier coat she's ago when I first got this boat so that's definitely going on.
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