What to do about tiny gelcoat blisters?

[Ryban]

We hauled our 40-year-old Westsail 32 out for the first time, and while removing all the old paint and an old failing epoxy barrier coat (it was just peeling off with a putty knife in a lot of places) we have identified many small blisters, almost all equal to or less than the size of a pencil eraser.

How should these be dealt with? There are 100s of them, and I’m not sure if they should all be individually be ground out, dried, and filled. Others have made the notable observation that, if this is the extent of blisters after 40 years, then we probably shouldn’t lose much sleep over this.

[s/v Jedi]

Originally Posted by Ryban

We hauled our 40-year-old Westsail 32 out for the first time, and while removing all the old paint and an old failing epoxy barrier coat (it was just peeling off with a putty knife in a lot of places) we have identified many small blisters, almost all equal to or less than the size of a pencil eraser.

How should these be dealt with? There are 100s of them, and I’m not sure if they should all be individually be ground out, dried, and filled. Others have made the notable observation that, if this is the extent of blisters after 40 years, then we probably shouldn’t lose much sleep over this.

If I understand correctly you are not DIY for this? If so then you need to get someone to check this for you. If you can DIY then the first thing is to investigate a couple of these. You need to find out where the void is… it can be right behind the barrier coat for all we know now. Take a small grinding tool (dremel is fine) and open the blister. Does fluid come out? What does it smell like? Do you see a small wet opening where the fluid comes from? If so, follow that channel by grinding deeper until it stops. Now you have found the source position of the blister. Repeat it for more blisters in other areas to verify the results. Now there’s information to decide on the cause of action.

Worrying that the barrier coat comes off. This means the prep before applying it wasn’t done properly. I recommend to strip it and do it again. After stripping, scrub the hull with soap and pressure wash. Then do a full wipe with denatured alcohol just before applying the first coat.

[Ryban]

We're doing this all DIY. I suspect the original epoxy barrier coat was applied by the PO many years ago. When we first hauled the boat out and started inspecting, we saw some quarter sized blisters, but upon inspection of these, these turned out to be just under the epoxy barrier coat and above the gelcoat. In one spot, we even found a worm living under the epoxy barrier coat. In many places, we were actually able to use a putty knife to remove large sections of the old epoxy barrier coat as I don't believe it ever formed a strong bond with the underlying gelcoat -- possibly due to bad surface prep given the underlying gelcoat is pretty smooth, but again, the old barrier coat could be 40 years old at this point.

But after revealing the underlying gelcoat, there are also these very small gelcoat blisters as in the photo above. I've grinded a few of them out and they don't seem to go into the glass much at all, if at all. The glass seems dry and is not weeping moisture after the initial penetration.

I also took a core sample of the hull because we are adding a new thru-hull anyway, and weighed it. I then have been letting the core sample dry for a week and also in our food dehydrator (heat and forced convection), and the weight has not changed more than 1%. So I suspect, at least on this part of the boat, the moisture content of the glass itself is pretty low.

[mvweebles]

Pencil head sized blisters are more than likely raised surface - gel coat. Personally, id sand them roughly flat and just go from there. You don't have a high performance boat. Likelihood of any structural issues is as close to zero as I can imagine.

Personally, I think folks get way too worried about bottom blisters. And this coming from a guy who's owned a Uniflite (went out of business due to class action lawsuit on blisters) and two Willard's, one of which had a decent crop of largish blisters.

[s/v Jedi]

Originally Posted by Ryban

We're doing this all DIY. I suspect the original epoxy barrier coat was applied by the PO many years ago. When we first hauled the boat out and started inspecting, we saw some quarter sized blisters, but upon inspection of these, these turned out to be just under the epoxy barrier coat and above the gelcoat. In one spot, we even found a worm living under the epoxy barrier coat. In many places, we were actually able to use a putty knife to remove large sections of the old epoxy barrier coat as I don't believe it ever formed a strong bond with the underlying gelcoat -- possibly due to bad surface prep given the underlying gelcoat is pretty smooth, but again, the old barrier coat could be 40 years old at this point.

But after revealing the underlying gelcoat, there are also these very small gelcoat blisters as in the photo above. I've grinded a few of them out and they don't seem to go into the glass much at all, if at all. The glass seems dry and is not weeping moisture after the initial penetration.

I also took a core sample of the hull because we are adding a new thru-hull anyway, and weighed it. I then have been letting the core sample dry for a week and also in our food dehydrator (heat and forced convection), and the weight has not changed more than 1%. So I suspect, at least on this part of the boat, the moisture content of the glass itself is pretty low.

Yes, they look like gelcoat blisters. I do recommend to deal with them because they are so small, it will be a quick job. You need to find a tool to quick grind them out, which is most of the work. An angle grinder is too big.

I recommend this combination, which worked for me for similar tasks:

https://www.amazon.com/Milwaukee-Rig.../dp/B0C6L3752Z
with
https://www.amazon.com/gp/product/B08BCCT6F4

When they are all open, pressure wash and dry, then go over it with a pair of putty knifes (I was taught to use a large one to hold the compound and a small one to apply) and TotalFair epoxy fairing compound. Sanding together with the rest if the hull before the new barrier coat goes on. I would first put a little barrier coat on just the TotalFair spots to seal them.

[Don C L]

Are these blisters causing a problem? Are they leading to a problem? Are they slowing your boat down? If you hadn't seen them, would they make any difference to your boat's seaworthiness? I doubt it. YOu could go to a lot of trouble to sand them all down, do some filling and priming. Or you could just paint over them and await until next haulout to see if they have changed at all. I bet you $10 they'll not have changed.

On my old boat I had a bunch of those tiny things. I sanded them and filled them and primed and painted. I felt better that my boat had a smoother bottom and I felt like the boat went a little faster too. But I have no empirical evidence to back that up.

Do what makes you feel better IMO.

[s/v Jedi]

Originally Posted by Don C L

Are these blisters causing a problem? Are they leading to a problem? Are they slowing your boat down? If you hadn't seen them, would they make any difference to your boat's seaworthiness? I doubt it. YOu could go to a lot of trouble to sand them all down, do some filling and priming. Or you could just paint over them and await until next haulout to see if they have changed at all. I bet you $10 they'll not have changed.

On my old boat I had a bunch of those tiny things. I sanded them and filled them and primed and painted. I felt better that my boat had a smoother bottom and I felt like the boat went a little faster too. But I have no empirical evidence to back that up.

Do what makes you feel better IMO.

Following that idea, why put a barrier coat on at all? A barrier coat isn’t effective with hundreds of blisters behind it and breaking through it during sanding.

When you don’t have to redo the barrier coat, there are good reasons to let it go for another year or more, but when you have to sand and apply a new barrier coat, it is silly to not address the blisters which will kill the new barrier coat again.

Do it right or do it twice.

[Ryban]

Well as it turns out, we found much larger hand-sized blisters further down on the keel, where I presume the lay-up was not done quite as precisely given the tight working space, and was probably resin rich. These presented as both large slight raised pockets, and also responded with a dull thud from the hammer.

I think there are only a dozen or so blisters like this, but still unfortunate, and might throw a wrench in our plans to be back in the water any time soon (if we want to barrier coat). We'll definitely grind all these out and repair them though.

The core sample I took, just above the keel section of the hull (where I added a thru-hull) does not seem to have more than 1-2% by mass moisture, but I imagine the lower section of the keel is a different story.

[s/v Jedi]

Originally Posted by Ryban

Well as it turns out, we found much larger hand-sized blisters further down on the keel, where I presume the lay-up was not done quite as precisely given the tight working space, and was probably resin rich. These presented as both large slight raised pockets, and also responded with a dull thud from the hammer.

I think there are only a dozen or so blisters like this, but still unfortunate, and might throw a wrench in our plans to be back in the water any time soon (if we want to barrier coat). We'll definitely grind all these out and repair them though.

The core sample I took, just above the keel section of the hull (where I added a thru-hull) does not seem to have more than 1-2% by mass moisture, but I imagine the lower section of the keel is a different story.

That seems to be down below the first layer of woven fiberglass which is structural, to I recommend to sand it out wider, beveling the edges, then add a patch of 1708 with epoxy resin. After cure, scrub with water, wipe with denatured alcohol and fair it out for the new barrier coat.

For these tight spaces, I like to use a thin piece (5mm) of plywood, large enough to reach the unaffected surrounding surface and flexible enough to follow the curve and use that to “screed” the epoxy fairing compound. You can put some peel ply over that and roll epoxy resin over it. When done right, the repair is sealed and requires no further sanding.