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Old 18-05-2003, 18:39   #1
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Osmosis repair

Has anyone ever heard of using dessicants to speed the drying time for osmosis blisters (in the laminate) in humid climates. I have several blisters in my hull and need to make the repairs in Hong Kong, where it is very humid most of the year. We use dessicants here as closet-size dehumidifiers, which got me thinking that if one could somehow slurry the hull with this chemical, it might help dry things out.

Thoughts?
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Old 18-05-2003, 18:50   #2
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Dessicants

Several things come to mind:

Pressure water wash the peeled or opened blisters daily at first, then less often. This disolves the entrained salt, and actually aids the drying process. The condensed salts, within the laminate, are hygroscopic, and must be flushed-out.

Alcohol (isopropyl, methyl hydrate, or gas-line anti-freeze) has an affinity for water. Wipe /w lots of alcohol-soaked paper towels, changing towels often.

Tent or skirt & ventillate and/or dehumidify.

Ethylene-Glycol (anti-freeze) is used to preserve wet wood. I'd like to hear opinions on it's utility with FRG. It may be a very BAD idea, or not!

Simple hydro-test. Tape "Saran" wrap over the gouged/cleaned blister. If vapour condenses inside, it's still wet.

Regards & good luck,
Gord
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Old 18-05-2003, 20:50   #3
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The ethel glycol (antifreeze)is a bad idea as it leaves a residue that might impede getting proper adhesion. Do a peel or sandblast the bad laminate off. Grinding tends to heat up the surface of the laminate and pretty much guarantees a failure after the repair. Then tent the boat with a dark colored cover, make sure the bilges stay dry and run a dehumidifier in a climate like Hong Kong. In a month or two you should be dry enough for the repairs. Use either epoxy or Vinylester and do at least one layer of fiberglass cloth. Keep the filler and fairing materials as thin as you can and make sure you do a 3 or so top coats over the least layer that you sand agressively.

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Old 19-05-2003, 11:54   #4
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Osmosis repair

I just hope 2 months will do it ... I have heard horror stories about 4-6 month dryouts, especially in humid climates. I've seen the enclosure/humidifier method employed around here, but not sure how quickly it works. I'm planning to lift over winter (ironically, the best sailing of the year!), which should minimize some of the humidity - but that means it might be 70% instead of 90%.

So, no thoughts on using dessicants? I realize it would be a novel approach, but shouldn't it work in theory?
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Old 19-05-2003, 12:30   #5
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Dessicants are pretty ineffective when compared to a dehumidifier. Even a pretty mediocre dehumidifier can remove a gallon or more of water per day. I cannot imagine a reasonable quantity of dessicant that can come close to that. Even if it could you would be drying a gallon of water per day out of that dessicant by baking it dry. Get a dehumifier, its cheaper to buy and cheaper and easier to operate.

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Old 07-06-2008, 18:07   #6
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In 1990 I repaired 8 blisters on my sailboat. They were 3/4" - 1 3/4" in diameter. There was a fiberglass specialist in the yard and said he would work on a time and material basis and let me do the basic stuff as he advised.
We sanded the blisters open and down to clean laminate, with a tapered edge. Each resulting crater was about 5" in diameter. We used about a 40 grit disc. We let it dry out for 5 weeks (in Florida) he checked the hull every week with a moisture meter.
He told me to wipe each crater down with acetone a few times each day.
when doing the repair, he had me cut a bunch of different sizes of fiberglass material. I think it was mat. He coated the crater with epoxy and laid in a piece of fiberglass, rolled it and repeated, using a larger piece each time, until the crater was filled.
The next day he came by and put on a fairing coat, and it was done.
Of course a week later when we moved a stand to paint under it, we found another blister we missed!! Damn!
I had him look at it and told him we had to be leaving soon. He told me to grind it out and hit it with acetone every few hours, and 2 day later he would mix up some "hot" epoxy and repair as he did with the others. he said it would probiblly be ok.
We cruised full time for 6 more years, and then 2- 4 month cruises after that, and none of the blisters ever came back. The boat did develope maybe 2 or 3 new blisters, in the following years.
I would never consider removing the gelcoat and doing an epoxy bottom, unless it was real bad.
I know a friend who has had his Hans Christian 41 botton pealed and done 3 times!!! True story.
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