osmosis

[tyce]

i have been removing the paint ready to epoxy it on my hunter horizon 272 (1989), and unfortunately i have found a lot of blisters.
the blisters are no more than 6-7mm diameter i could not see them until all paint was removed and i was sanding even then i could only see the outline,so they arent raised and are flush with the gel coat, just the outline is visible after sanding, when you press against them i can hear a cracking noise and some let out a bit of water.
i assume this is the dreaded osmosis,and having never seen it, i was hoping you could give me some advice, is it osmosis or wicking, is it wise to epoxy coat it now, or am i better of monitoring it, and eventually having it repaired, and at what state is it deemed vital i repair it, would you class this as very mild osmosis and could i get away with epoxying it

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[pappaecho]

If you prick a blister it should have a vinegary sort of smell, which is a sign of the dreaded osmosis. However before you panic, bear in mind that no boat has ever been recorded of sinking due to it.
I suggest that you speak to the technical department of Blakes Paints ( hempel) in Southampton, as they will give you a complete action plan for the filling of the blister prior to epoxy overcoating

<hr width=100% size=1>Def: Yachting - a way of spending the kids inheritance

 

[Robin]

Don't do anything without proper advice. IMO you certainly cannot proceed with an epoxy paint job as originally planned, it will merely seal any water in. If you cannot or don't wish to do a proper job at this time then do localised repairs where possible and re-antifoul the full job will be the same next year or whenever you chose to do it. If you do epoxy over a wet hull then the proper job will be that much harder to do.

I would consider asking for some professional advice from a surveyor with experience of osmosis repairs. Your hull may not have true osmosis, it may be wicking but it matters little, the cure is likely to be the same which is to peel/grind/gritblast the gel to open things up and then dry the hull before doing a proper epoxy job, ie using solvent free epoxy and thick coatings, a couple of coats of solvent based Gelshield is like p****ng in the wind.

This isn't life threatening either, so you could even wait until you sell the boat and just take a hit on the price, advertise it openly priced to take the problem and it's cure into account.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[pelissima]

Tyce,
I m afraid u have no choice but bite the bullet and do some reading. A search in this forum will bring up loads of usefull hints and info. I cannot help though noticing that toilet papers attract over 30 enthousiastic replies, while yours is still slim.
On your question, epoxing now is a no no. At this moment, monitoring is the order of the day.Have patience

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[rex_seadog]

Probably not helping Tyce's dilemma but can someone explain how the "peel/grind/gritblast the gel to open things up" osmosis treatment actually works. Does this mean that all the blisters are near the outer surface? If not I have horrible visions of going straight through the hull when trying to open up some of the inner blisters. In fact I have read that leaving water in the bilges can cause osmosis, presumably from the inside.

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[pelissima]

I could not agree more with you on Gelshield.
I wasted good money on it back in 2001, as I was not reading this forum. Now I have to go through the process again.
This time I will go West or SP.

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[Robin]

There is nothing wrong with Gelshield per se, just that the stuff you buy in the chandlers is a solvent based version and like all solvent based paints once the solvent evaporates the remaining dry film is thin (about 50 micron I think). Even epoxy is not impermeable, it just takes longer than with gelcoat, so you need thickness to make it effective over time. At 50 microns dry film you might need 12 coats to give a lasting thickness but I believe International don't reccomend more than 5, because I believe of the risk of solvent retention between coats another problem!

We prevention epoxied our last boat with surveyor supervision, having had her grit blasted to open up any cavities (there were very few), to give a better drying surface and a better key for the epoxy. We then applied Blakes SFE200 solvent free epoxy at a measured thickness of over 200 microns per coat, 850 microns in all. Seven years on the sale survey gave her a very clean bill of health.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[Robin]

This is a huge subject! There is a book by Tony Staton Bevan that is very good and some websites, but really not for this post and diverting away from Tyce's question? Short answer is yes/unlikely/yes sometimes. Key is that it is not terminal and can be solved.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[Evadne]

Osmosis is caused by water passing through an osmotic membrane, in this case the gelcoat. Wicking is water passing through a small aperture by capillary action, and I've heard it applied to water getting in through pinholes or cracks (which I had) or what happens to the water once it gets in, i.e. wicking down fibres that haven't been fully soaked in resin. As has been said the net effect is the same, i.e. liquid water under the gelcoat.

The object is to get the water out before it causes structural damage, and received wisdom has that the best way is to remove the membrane (gelcoat) and dry out the rest with gentle heat. Then you can fix the problem. For this boat I guess you're looking at £3k or so to have it done professionally.
Other fixes (or bodges as others will call them) seem to be based on the idea that the water isn't that harmful anyway, so gouge out the blisters, fill and fair with the product of your choice. Repeat as necessary. Only time will tell who is right, but when I had my boat done I decided that method (a) was, at worst, unnecessarily meticulous and expensive, and method (b) was, at best, adequate and at worst, suicidal. As I (or rather Evadne) had wicking deep into the fibreglass, I'm glad I took the expensive option.

If you had osmosis from water in the bilges, then you'd expect blisters in the bilges. If water gets in from the bilges I'd have thought it was by wicking, which can be prevented with a coat of bilge paint.

And that's without opening any more cans of worms such as moisture meters and what are they actually telling you?


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[macd]

At the risk of half repeating my post of the other day, my understanding is that osmosis (or whatever you choose to call it) exerts a pressure between the laid-up membranes of the hull. This pressure is far too small to break down the bond between layers of mat, but can force apart the weaker bond between gelcoat and underlying tissue layer, or the tissue from the outer layer of mat, particularly if the original lay-up was not done properly. Neither of these would threaten the structural integrity of the hull.

Check out http://www.yachtsurvey.com/blisters.htm and related links. Good stuff, I think.


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[longjohnsilver]

Lots of good advice so far. The vinegary smell is very distinct, the fluid is a browny colour and this is caused by the water being attracted by any remaining styrene from the original lay up. It is vital to get rid of this. probably best way to proceed is to have the hull slurry or grit blasted, but essential to get a grp hull specialist blaster to do it otherwise lots of damage could ensue.

having done this the hull needs to be thoroughly washed with fresh water, best with a pressure washer but not with a pinprick jet, use the widest setting. Repeat this 2 or 3 times. Then the hard bit is allowing it to dry sufficiently before epoxying, you'll need a moisture meter for that. The drying process could be pretty slow particularly outside in a British winter! Also you'll need to fair off the hull with an underwater epoxy filler - this is time consuming!! Then epoxy, but make sure each stage is complete before moving on otherwise you could have to start from scratch, not a pleasant prospect!

Good luck!

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[Birdseye]

The boat's worth at least £15k and a surveyor costs £400. Best get some professional advice rather than rely on the mixture of hearsay, guesses and half understood technicalities that you will get on this forum. That way you have somebody to come back to if it goes wrong - in theory at least!

I've dug into blisters on a Sadler that were mushy grp for 3 to 4mm deep into the laminate. Hunters are better built but still not heavy boats with thick laminates, so dont ignore the problem on the "wont sink" argument. It is progressive - it wont stop until you do something about it.

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.

 

[Mudplugger]

Tyce, go with Birdseye on this one if you are really concerned!, However what hasn't been mentioned hear is Solvent burn (where unsuitable solvent has reacted with the gel-coat) it gives a similar appearance to the dreaded lurgy, BUT DOES NOT have the vinager like odour!. Probably worth opening a couple up and seeing what state the underlying laminate is in. There are Epoxy paints on the market that will hold back Sulphuric Acid... So don't panic...

<hr width=100% size=1>Tony W.

 

[Marsupial]

Tyce

There is a lot of myth and legend surrounding osmosis. My advice is, be very sceptical about “professional” advice, especially if comes from a company that has demonstrably invested thousands in paint shop technology. (IN MY EXPERIENCE watching and listening to such advise at boatyards) Its usually akin to the car workshop receptionist who suggests that unless you strip your car down to metal all over and repaint that stone chip on the bonnet it will cause a massive structural failure next week. Do some reading, do lots of research and find out how “the trade” deals with osmosis. They use terms like “knock down the blisters, rinse dry and fill” not strip off acres the gel coat and reapply. The difference in cost is massive; the quality of the finished job is no different. It is usually a very viable DIY job.

Wicking is a different thing, my understanding is that usually there are no blisters, as such but the surface of the gel coat looks like its been applied over lots of strands of cotton, or hair. It is of course a fault in the lay-up, the strands of fibreglass were not properly coated in resin and the unsealed ends have picked up water through the gel coat - like a wick and expanded. If it’s bad you have no choice but to strip off the gel coat. But monitor wicking, you may find it gets no worst over many years and hence poses no threat apart from a cosmetic one.

I hope this helps

David



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[cliff]

Re: osmosis / Gel shield

Could not disagree with you more. Gel shield does what it says on the tin - if properly applied. Why blame the product? more than likely and faults are a result of poor preparation or improper application.

International paints have released a CD covering all aspects of the use of their products - well worth picking one up from your local swindelery - the CDs are FREE by the way - just in case your local swindelery tries to charge you for one.

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[tyce]

thanks for all advice,
the blisters when pierced have no smell and the liquid is just clear water, the blisters arent what id call blisters as they arent raised and i thought they would have to be, they are only visible when paint stripped right back and when i sand them they arent visible. does anyone know for sure if this is osmosis.
thanks and keep the useful advice coming.

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[cliff]

"Best get some professional advice rather than rely on the mixture of hearsay, guesses and half understood technicalities that you will get on this forum"

Never a truer word spoken (or written) especially when comments such as Hunters were better built then Sadlers are posted with an air of authority.

Some people might actually believe such defamitory remarks, mind you I trust there are still some folk left that are capable of looking at the big picture.



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[cliff]

"I cannot help though noticing that toilet papers attract over 30 enthousiastic replies, while yours is still slim."

Perhaps forumites are tired of the same old subject especially one that has been hammered to death on so many occasions.

There again perhaps other forumites have seen the light and realised that osmosis is not as serious problem as some would like it to be and also realised that a good choice of toilet paper is more important.

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[Birdseye]

The osmosis on my boat was only visible as crescent shaped stains of antifoul left behind when the scraper lifted over the blister surface. You couldnt see the blisters themselves or even feel them, but that's what they were. They dont have to be big dinner plate sized jobbies at first.

You can tell whether the fluid coming out is acid or not by using litmus paper available from the chemist. If not try just the tiniest bit on the end of your tongue , but dont swallow and wash your mouth out afterwards.

You can carry cynicism about commercial outfits too far. Most people are honest in my experience, and if you get the same advice from all the tech helplines of (say) West, Blakes and International, then you can be reasonably confident in what you have been told. But in the end your best resort is a surveyor who has seen it all before, seen the cures, knows which repair shops do a good job and will (if you want to take that route) supervise you in a diy job. He will possibly even confirm what I said about Sadlers!

Good luck. Dont worry about it. Think of the poor sods in the Sudan!

PS From personal experience, the time taken to cure was as much a problem as the cost.

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.

 

[Marsupial]

Tyce
#
It is certainly the early stages of osmosis, i.e. water has passed through a membrane – usually the gel coat and is trapped between the gel coat and lay-up. If its still “water” then it hasn’t been there long. As it leaches chemical from the resins surrounding it and as more water joins it pressure builds up and pushes the gel coat out (because its thinner than the lay-up) and blisters form, the liquid then smells a bit like vinegar.

I have seen this treated in a number of ways ranging from

1. A complete peel of the gel coat, fresh water rinse until the hull dries (strange but true). Explanation – if there is too much acidic liquid in the repair the hull wont dry, if the hull wont dry its pointless reapplying gel coat or any other preventative resin for that matter.

2. Knock the blisters down with a ball pene hammer, wash the blister with a pressure washer, let it dry out and treat.

3. Grind the blisters with an angle grinder; wash the area with a hose or pressure washer let it dry and treat with new gel coat and or resin.

4. Ignore

There is a pattern here, all the methods rely on the repair being clean and dry, some sort of moisture meter is a must.

The amount of time you have to wait for the hull/repair to dry out is critical, if there is an advanced chemical action going on it wont dry at all so you have to keep diluting the chemical in the repair by washing with clean water – it can take months and many washings.

May and June are often lousy months for sailing, but warm enough to dry out hull repairs.

I am not sure if no boats have sunk as result of osmosis but that is my belief, I certainly know of boats that have had “galloping osmosis” for 25years and they are still afloat, one had blisters the size of hens eggs on the bottom but the owner just anti-fouled them every year!

Hope this helps




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