Thread: KK39 Blisters
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Old 10-11-2021, 03:06 PM   #18
Steve DAntonio
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Quote:
Originally Posted by Donna View Post
Show me an older boat that doesn’t have blisters. Southerly just went through a haul out survey. Some blisters that can easily be taken care of on her next bottom paint. 4” hull. No concern.
While it's true many older (and some new) vessels suffer from blisters, I have hundreds of photos of old bottoms that are free of blisters, it is by no means a forgone conclusion for an older vessel.

Osmotic blisters occur for specific and well-understood (unfortunately not by enough folks in the marine industry) reasons, primarily having to do with the presence of water soluble materials and water permeable polyester resin. Vessels that use vinyl ester, vinyl ester skin coats or epoxy bottoms are essentially blister-proof. Epoxy barrier coats, if properly applied, can also provide years' of protection from osmotic blisters.

There are blisters and there are blisters. I routinely encounter blisters on vessels adjacent to thrusters and shaft logs, those are the result of using fairing compound that was not rated for below the waterline use. It's not a serious problem and relatively easy to fix.

Wholesale, bottom-wide blisters on the other hand are often the result of classic osmosis issues. Unless the blisters are very large and very deep, it's unlikely they will pose a structural threat, but it's impossible to make this determination with a visual analysis alone. They will have an impact on the vessel's resale value unless you pay to have them repaired, and doing so correctly, by peeling and relaminating with VE resin, is costly. Spot repairing blisters, i.e. grinding and filling, is addressing the symptom and not the cause. Spot repairs are a cheaper option, but more blisters will almost certainly continue to reappear.

I would not necessarily walk away, but you should have the bottom assessed by someone who is very knowledgeable on the subject, preferably one who has undertaken repairs. Only mildly destructive testing (doing what's known as a "patch test") can provide an accurate analysis and quote for repairs.

For more on analysis, repair and prevention techniques, see https://stevedmarineconsulting.com/w...Jul_Aug-06.pdf
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