Re: Unexpected blister

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Posted by B.B. on February 14, 2005 at 03:22:36:

In Reply to: Unexpected blister posted by John Musgrave on February 14, 2005 at 03:18:58:

Posted by Bengt Blomberg on November 25, 2002 at 11:49:32:
In Reply to: unexpected ?blister posted by John Musgrave on November 24, 2002 at 06:36:30:
Hello John,
Much of the Beneteau production from those years vere affected by hydrolysis due to manufacturing faults even if less so for the smaller sizes.
An epoxy coating on a hull where the causes for the problem have not been removed will very much accelerate the problem and damages. Also the blisters are the visible sign only that there is a hydrolysis problem and if there is a voven roving layer of 600g or more among the first thre or four laminate layers, this layer can delaminate sufficiently to swallow the hydrolysis products and no blisters will form.
There can be two reasons for the blister now detected: 1. There is a capillary from such a delamination to the surface layer where the pressure earlier could seep through the gelcoat without causing a blister but where the less absorbing and more elastic epoxy now blisters.
2. The first epoxy coat might have formed a carbonate skin before the next layer was applied. Then there will be "delaminations" between the eopoxy layers where moisture enters and reacts with the carbonate forming alkali and pressure which causes the blister.
It is very easy to determine which is the cause.
Buy some lithmus paper in a drugstore and check the pH level of the blister fluid. If it is acid (below pH 6.5) it is a hydrolysis problem. If it is alkalic (over pH 8) it is an epoxy carbonation.
In the later case I would recommend to remove the blister and eventual later blisters by sanding and nothing more. A polyester laminate has absolutely no need for any "protective" coating other than for scratch protection. The gelcoat never had any moisture protection purpose. It is a cheap way of providing a long lasting colour coat only.
If the fluid is acidic I can recommend a HotVac treatment only. It has proved to be fully or even more effective than the much more expensive HYAB treatment which earlier was the only reliable one.
Even here an epoxy coating is not needed for moisture protection but as polyester filler does not stick properly to old polyester surfaces, epoxy filler must be used to restore the hull surface and then some epoxy coats on top is logical for scratch protection.
Regards Bengt

: Posted by John Musgrave on November 24, 2002 at 06:36:30:
: This spring I got ambitious and removed all previous antifouling from the bottom of my Beneteau 26, 1984. I do not think it had ever been done before. Then I rolled on three coats of West System epoxy and congratulated myself for having protected the hull better than it had ever been. BUT on hauling this October, to my chagrin, I found a single lesion, 3 cm diameter, elevation maybe 3 to 4 millimetres, in the bow section, which exuded some clear fluid through what seemed to be a pinhole sized hole in the gelcoat. Is it likely that a hull previously free of osmotic blisters could have suddenly started to develop them, and if so, could the epoxy sealing have accelerated the process (as I have seen suggested somewhere)???

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