DIY repair of localised osmosis blisters

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Longshanks

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Just found a little group of osmosis blisters beginning to form in one part of the hull. I suspect the moisture may have come inside rather than outside through the epoxy coating and with a bit of luck is limited to this small area only, so I'm going to tackle this myself. I think there's a PBO article on this somewhere, but what does the forum think? My plan is to grind out the individual blisters, rinse well with water to get rid of any solutes, let dry for a day or so and then fill the craters with epoxy filler. Top coat locally with more Gelshield then keep a good eye on the area over the next year. Who else has done something similar? Did it work?
 
The process you propose is essentially sound. Your drying period is too short though. Wait until you haul out in the autumn, dig out all the guff, wash liberally (pressure washer) every day for a week. Then tape a plastic 'skirt' around the hull on the waterline, allowing the wind to really dry the hull out. Leave until spring, then fill and fair and finish with VC tar or equivalent.
Nicki
 
As Nicki suggests it is no good expecting things to happen in a day...You will smell vinegar whaen you puncture the blisters and you need to wash at least dailey if not twice dailey till the smell goes. This could take weeks or even months.

Then as others have said fill and fair and paint with vc tar or gelshield etc.
 
Probably the best idea Lakesailor. I've spent a great deal of time arseing around with blisters and I have no idea whether any of it has been fundamentally worthwhile
 
Random small blister sites are difficult to eradicate and further eruptions can occur due to continuing hydrolosis iIf blisters are smaller than thumbnail leave well alone and moniter.
 
I only found them after removing the antifoul, so they're not exactly large, numerous and obvious! I'm also wondering whether to just leave them and see if they become a problem (or not). I'm sure others would just smooth them off and re-paint. I'll take the advice and rinse / dry for as long as I can before re-launch this year, but it won't be many months unfortunately. If they do come back, the effort is not that great, so nothing much lost if I have to re-do them next time. They are adjacent to the chain locker, and I wonder if many decades of chain abrading the inside of the hull, plus a generally damp/wet poorly drained location has led to water absorption into the laminate from inside? Anyone else seen a similar problem?
 
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Then tape a plastic 'skirt' around the hull on the waterline, allowing the wind to really dry the hull out. Leave until spring
Nicki

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Isn't using a plastic skirt supposed to enclose the hull so that damp air is not circulating round it. Also that you can use dehumidifiers to extract moisture from the air and in so doing extract it from the hull. Surely doing as you suggest is wasted effort as you let wind in carrying all sorts of moisture. In fact with that, you could completely forget using any plastic skirt.
 
If the boat has been epoxy coated previously, carefully remove the epoxy first before examining the gel-coat. They may just be in the epoxy. If not I agree that local repair now is not a problem and will indicate if you need to do more at a later stage.
 
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Who else has done something similar? Did it work?

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I decided to approach our blistering in this way 5 years ago. I decided against a full pro 'repair' after having looked at several boats mid-treatment in the yard: it was just far too agressive for me, with far too much good laminate being planed away and not replaced.

Small (5-7mm or so) blisters/pimples were ground out using a blunted countersink bit in a drill. Cleaned, dryed and filled with epoxy + silica.

Anything larger (and there were many!) ground out with a flap disc + angle grinder. Cleaned, dryed, laminate rebuilt, filled with epoxy + silica, faired with epoxy + West low desity filler.

Although all the above advice about repeated washing and drying is of course correct for major osmosis treatments, I'm not sure it's that relevant to localised repairs like this: your not going to get the results of hydrolosis out of the laminate through small blister craters. Just wash them out, dry for a day or so in the sun then repair would be my advice. You will have already made the decision to live with a 'wet' hull.

5 years on there's nothing new and nothing has re-occurred.
 
All the skirt needs to be is a bit of two or three inch plastic taped above the ground out area to deflect rainwater running down the hull side - you're not trying to enclose the whole hull - you want fresh air getting to the exposed bits.

If the blisters are localised I'd say grind out, wash, dry out, and refill with epoxy, but preferably dry for longer than a few days, and more than one washing (with hot water). If the blisters are few/small area you may as well do it: you may well get more but probably in different places next time. The work is not that difficult or time-consuming. Different if the whole hull is covered with them, but even then leaving alone is an option....
 
Assuming the boat is outside and not in hugely expensive heated accommodation, if rain is kept off the affected area, the wind does a super job of drying the hull. If the skirt entirely seals the hull from outside, there is nowhere for the water to go. It needs to be left clear of the ground, or, as suggested elsewhere in this thread, placed locally to the repair. There are many ways of skinning this particular cat, but the key is removal of the water (solvent) present in the hull, and the subsequent re-coating to keep to keep the laminate as dry as possible.
 
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If the boat has been epoxy coated previously, carefully remove the epoxy first before examining the gel-coat. They may just be in the epoxy. If not I agree that local repair now is not a problem and will indicate if you need to do more at a later stage.

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I've only seen one or two blister domes in the epoxy (and very shallow). Mostly what's happened is that the scraper has exposed a light coloured circle about the size of a fingernail or smaller in the grey epoxy coating (presumably taken the top off the blister) That circle is full of a powdery deposit, which on further prodding exudes a small amount of fluid. This does have an acetic acid smell. I know the hull was sandblasted before being Gelshield coated many years ago, but it's possible I suppose that some areas of original gelcoat were not fully removed and therefore not properly washed / dried. 99% of the epoxy coating seems fine. I'll be going over the hull with a moisture meter next week to build up a picture of what's going on.
 
Steve_N...thanks, that's encouraging!

PS to all - the epoxy I'm told is Gelshield and looks grey with a hint of green. Is this likely to be the grey version? I think the green version is actually quite an obvious green from what I've seen on others' boats. Does anyone know?
 
Could be the old solvent free Gelshield where the grey was made by adding a sachet of aluminium pigment to the green base meaning you had a grey/green result.
I go along with the people who say leave it alone or, at most, do a simple localised repair. If you do repair it is worth doing a reasonable job by grinding back in the area where blisters occur, washing several times and allowing to dry for a period. If the blister fluid smells of acetic acid you are unlikely to cure the problem by just opening individual blisters. Remember when you fill and epoxy coat to use solvent free products, at least for the first coat. Best to apply 1 epoxy coat before you fill to seal the laminate and prevent any air pockets, then fill and finish with several more layers of epoxy
 
I would agree with Lakesailer. I worried at one time about osmosis, then started reading around proper web sites - not those produced by prophets of doom who want to sell expensive treatments. The questions which are relevent are:

a) Do you want to sell the boat? If so, and it has a real value, then you will have to weigh up the cost of treatment against the value lost.

b) How long are you going to own the boat? I would suggest that the Osmosis will not sink it, or cause any problems for many decades, particularly if the boat spends 6 months of year out of the water.

c) Do you want to sail it and enjoy it, or waste time scraping and grinding?
 
Thanks for the info Pasarell. I think you may be right about grey/green Gelshield. The area with multiple blisters is very small and should be ground right back (probably next year) but elsewhere the blisters are completely isolated (and very few), so I'll fill these and hope that's an end to it. Only a few smell of acetic acid; others don't.
 
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