Are gelcoat 'osmosis' blisters structural or cosmetic damage

[tidalwave]

Before I bought my 26M, it was kept for three months in a freshwater
lake marina without bottom paint...
The white gelcoat on the hull has several areas where there appears to be
multiple pin-head size 'osmotic blisters'
I plan on having the hull sanded and a epoxy bottom paint applied.
The ballast tank in the M does not appear to have any kind of special coating on the fiberglas.
Can the ballast tank then develop blisters?
If so...do the blisters cause any structural damage?

[Lorne Colish]

My developed blisters after one season unprotected in freshwater and was professionally sanded and coated with a clear epoxy before I bought it. I coated it with Aurora bottom wax and it blistered again. Upon investigation,I found that the Epoxy was the wrong type,not intended as a barrier coat.I sanded the the clearcoat off and applied 4 coats of Interlux Interprotect 2000-2001,followed by 2 coats of antifouling,and it has been good for the last 2 seasons.
My first Mac,a 1988 26D did the same thing and was cured with the 'Interprotect'.
Osmosis is more than just a cosmetic problem and can become quite destructive if left unrepaired.My experience tells me that any boat that sits in the water for extended periods of time should be protected.Both of my Macs were new when they blistered.There's lots of good info about this issue. In both cases I did the work myself.

[Frank C]

Lots of discussion in this earlier thread about keeping the Mac in a slip.

[Divecoz]

Ours sat in the water for several months last summer "naked as when we was formed " and no problems . I never had a problem with any of the ski boats and they always sat 6 months or more in fresh water at the cottage .

[Lorne Colish]

I also know of boats that have been in the water without barrier coats or antifouling for extended periods of time,many summers in some cases,and have not developed osmosis.But I can't explain why some get it and some don't. Having had 2 macs develop the problem has certainly affected my view.