Osmosis on my Sadler 29

[dickonclark]

Help please!
I own a Sadler 29 which is 27 years old, and have noticed two patches of small goose pimple like patches above the waterline!Is it possible to have osmosis above the waterline? And in the same place both sides! Do you think the foam sandwich construction may have anything to do with it? I'll get some photographs at the weekend and put them on my website so you can see.

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[bntii]

We call these "Gelcoat blisters" and are typically limited to the surface coating lifting from the substrate. That is- the laminate is not deteriorating and does not appear to be the source of the blistering.

Mechanism seems to be porosity of paint/gelcoat, surface contamination, solvent entrapment, some component of sub strata going into solution, or some mix of the above.
It can happen fairly rapidly on painted surfaces above the water which stay too damp & may suggest some lack of good keying of paint/primer to substrata.
It is common to see on hulls which have a well protected bottom and a trim issue which keeps some portion of the topsides submerged.
Other boats can be just covered with this damage to the gelcoat on the under body.

My guess- the sandwich (cored?) hull has nothing to do with it.

Originally Posted by dickonclark

Is it possible to have osmosis above the waterline?

I just put a repair bid in on one of the Valiant 40's with the fire resin- 1/4" deep laminate blisters the size of a egg fairly covered the boat all the way to the rail.
The bottom was worse...

[dickonclark]

When looking closely it looks as though there may have been a small paint job in the area, which would confirm what you are saying, however there is a slightly larger blister and when popped a yellowy substance came out! The strange thing is this all happened in the yard so I'm wondering if water has been running down the decks and over the sides at the same point both sides? Any ideas on a solution? Maybe another paint job or could I just sand them back and fill with gelcoat filler? Trouble is the white filler never colour matches.

Thanks for your help

Dickon Clark

[bntii]

My experience is the break point for "gelcoat blisters" or coating blisters is < ~1/4"
On observation I look first at size then the presence of discolored fluid in the blister.
As soon as I see fluid and increased size the blisters are more likely to be laminate blisters. In this case the mechanism is some component of the laminating resin going into solution and driving the osmotic potential. Laminate blisters are accompanied by some level of resin starvation in adjacent laminate. In the most common case, the laminate blisters are limited to the mat layers which are highly susceptible to blistering.

Solutions?

Is this a gelcoated or painted hull?
Gelcoat or coating blisters can be effectively sanded out and the repair primed and painted or new gelcoat sprayed on.
Laminate blisters are a tougher case but can be dealt with as well.

Take a pocket knife and carefully scrape off the top of one of the small blisters.
You are looking to access first- what the shell of the blister is comprised of. e.g.- is it a film of paint? And secondly, what surface that blister lifted off of. Do you see primer under the blister? A glossy pocket in the matt? A white spot in matt? Is there a dusty residue in the blister?

Photos would help when you get them posted.

[dickonclark]

Peeled off one of the blisters and I can see what looks like primer! Someone has suggested that maybe the extreme cold has caused it as I've never noticed it before.
Dickon Clark