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johnnyandjebus 10-23-2010 07:00 PM

Blisters, yes another blisters thread
Hello all

Apologizes for starting up a new tread on a topic that,I am sure, has been well discussed here.

Every year when the boat gets pulled out I find approx 20 small blisters on either side of the hull, approx 1/4 to 1/2 inch in dia. 90+ percent of them are water trapped between the hull and the anti fouling paint. I pop them in the fall and let them dry out before sanding and re-painting in the spring.
If I suspect that the blistering has penetrated the glass in anyway, the blister is touched with a dremmel tool, dried out, sanded, and then filled with resin. No filler used.

Approx 10 years ago the hull was stripped of all anti fouling paint and approx 8 layers of fiberglass resin was applied, in an attempt to pre-actively seal the hull. At the time there was not an issue with blistering beyond what was described above. Done in the spring not the fall.

My question;
What causes this blisters to occur in the first place?
Am I not applying the anti fouling paint correctly?
Not prepping correctly?



St Anna 10-23-2010 07:36 PM

I had something similar a few years ago.
The reason was the 2nd coat of barrier seal was put on outside the recoat time and did not bond. I stripped it back about 3mm and re-epoxied, barrier coated etc. It was by far an overkill solution but it is done now.

PorFin 10-23-2010 08:47 PM


You mentioned using "resin" twice in your OP -- once to fill/fair minor repairs, and once for a barrier coat. What type of resin are we talking about? Epoxy? Polyester? Vinylester? It does make a difference.

mitiempo 10-23-2010 11:09 PM

PorFin beat me to it but the resin makes a big difference. If the hull was coated 10 years ago with polyester resin it would not be more effective at stopping water absorption than the original material, also polyester most likely. Epoxy is the best choice for barrier coats (Interprotect is epoxy) and 5 coats or more are necessary to build up enough thickness.
As far as the bubbles in the anti fouling paint it could be causes by not prepping correctly.

johnnyandjebus 10-24-2010 12:15 PM

Hello all, thanks for the replies

To clarify a bit.

Approx 10 years ago the hull was stripped of all anti-fouling paint and then 8 coats of epoxy resin were applied, no filler added. This was done by my father and he can not recall the details has to how he went about doing this other than what is mentioned above.
Each spring if I find any blisters that appear to have penetrated the epoxy barrier coat, they are dremmeled and dried out. I sand the anti fouling paint away, approx 4 inches radius around the blister and apply epoxy to it, no filler. Usually 2 - 3 coats, sanding and feathering in between.

The vast majority of the blisters are water trapped between the anti-fouling and the epoxy barrier, which leads me to believe, as suggested, that I am not prepping the hull correctly before the anti-fouling paint is applied.

So, next spring, should I sand down the anti-fouling to bare hull and start with a fresh layer of anti-fouling?
My main concern is to get things done right, an ounce of prevention being worth more than a pound of cure.

I am currently using Interflux bottomkote XXX as my anti-fouling paint.


mitiempo 10-24-2010 12:22 PM

One thing I can think of is amine blush. The epoxy should be scrubbed with a scotchbrite pad and water then rinsed well before the anti fouling paint is put on. This might be the problem.

PorFin 10-24-2010 06:22 PM


Originally Posted by mitiempo (Post 658463)
One thing I can think of is amine blush. The epoxy should be scrubbed with a scotchbrite pad and water then rinsed well before the anti fouling paint is put on. This might be the problem.

Couple of things to add.

Brian's probably right. If the amine blush wasn't removed after that last coat of epoxy had cured, then this could be the cause of your water ingress.

If you want to dave yourself some time & effort next time you do the fair/fill kibuki dance, try using cabosil/colloidal silica as a thickener. You can probably completely fill your craters in one go.

If you elect to go without cabosil and go the multiple coats of unthickened epoxy route, don't let it fully cure before you apply the next coat. As long as it's still soft enough to "grab" the fibers in a cotton ball (or you can indent it with a fingernail), it's ready for the next coat of resin without further prep. This is actually preferable to letting it fully cure, since you'll achieve a chemical bond between the layers instead of a mechanical one.

If it gets too cured for the above, then let it go to an almost full cure. Before you have at it with sandpaper, like Brian said give it a good scrubbing with a green ScotchBrite and water, then rinse well. This will remove the blush that accumulates on the surface of the epoxy. Then sand it for the next coat.

BTW, if you've removed the blush from your surface layer of resin you won't need to do it again next time you sand/paint.

PorFin 10-24-2010 06:25 PM

I just noticed you haven't yet gotten an answer about surface prep prior to painting, so here goes:

Follow the instructions of the paint manufacturer. THey are usually very comprehensive, and often include use of proprietary prep washes that promote a good bond between the hull and the paint.

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