Osmosis ?

ghostlymoron

ghostlymoron

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Whilst prepping my boat for painting, I found many small blisters near the stern at the water line. These don't look like traditional osmosis as they are only about 3mm dia but what else can they be? I'm going to treat them as if they are, grind out, rinse, dry thoroughly with heat gun then fill, fair and apply paint system. I've only got general filler not 'Marine' grade, will this matter?
So out with the grinder today.
 
Davegriff said:
Serious question. I see this stated a lot lately, but why is it not exactly? My boat is made of polyester resin as are most fillers and its waterproof, having been afloat 30 odd years!
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Normal Plastic Padding and the like absorb water and can lose adhesion to the cured resin. Epoxy based fillers are widely available and adhere well to cured resin, don't shrink and are waterproof. Recommended for underwater applications.
 
ghostlymoron said:
Whilst prepping my boat for painting, I found many small blisters near the stern at the water line. These don't look like traditional osmosis as they are only about 3mm dia but what else can they be? I'm going to treat them as if they are, grind out, rinse, dry thoroughly with heat gun then fill, fair and apply paint system. I've only got general filler not 'Marine' grade, will this matter?
So out with the grinder today.
Click to expand...

I have something very similar sounding on my Hunter 490 - I asked about it in previous thread. A myriad of tiny little hemispherical pits in the gel coat, 1 - 3mm across, each of which squirted out enough acetone-smelling liquid to raise a blister, typically 6mm - 20mm across, in the paint above. From the smell it must surely be osmotic, but it's not at all like classical osmosis. I'm not sure if I can be bothered filling them, as they are spread across the entire underwater surface. I think I'll finish stripping (RemoveAll 620 did a pretty poor job) then paint with something epoxy and put up with a very slightly dimpled effect.
 
You probably have air bubbles in the gel coat, not osmosis. On my Fulmar I have plenty and Tony (Mr Osmosis) Stratton-Bevan when he surveyed her said that Westerlys were well known for this. Westerlys had a double gel coat, so air could be in either layer or between the layers. The air bubbles can be from 1mm to 5mm in size and they will never get any bigger and highly unlikely that water will reach the glass strands that would wick water in to the glassfibre to start osmosis.
P3011098.jpg

All I would do is sand the affected areas and then fill with an epoxy filler and then paint over. In my case I followed professional advice and gave the hull 2 coats of GelShield. This was only a preventative measure, but one that should provide benefit in the future. I was warned that more air holes could appear in the future, so it was not worth having a greater number of coats than 2. If you want to see the work I have undertaken then look at http://s1294.photobucket.com/user/ConcertoFulmar32/library/?sort=3&page=3.

Hope this information explains what has happened.
 
The other thing it could be (but I don't think it is in this case) is Gelshield separation. If gelshield is applied at a low temperture the mixture does not "go off" as it should and the components separate into blobs. When the temperature does rise to the curing temperature the blobs harden and look like blisters.
I agree with the other posts about using marine epoxy to repair hull defects.
 
SHUG said:
The other thing it could be (but I don't think it is in this case) is Gelshield separation. If gelshield is applied at a low temperture the mixture does not "go off" as it should and the components separate into blobs. When the temperature does rise to the curing temperature the blobs harden and look like blisters.
I agree with the other posts about using marine epoxy to repair hull defects.
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What do you mean by marine epoxy? I was planning to use Plastic Padding Marine Filler
 
Concerto said:
You probably have air bubbles in the gel coat, not osmosis. On my Fulmar I have plenty and Tony (Mr Osmosis) Stratton-Bevan when he surveyed her said that Westerlys were well known for this. Westerlys had a double gel coat, so air could be in either layer or between the layers. The air bubbles can be from 1mm to 5mm in size and they will never get any bigger and highly unlikely that water will reach the glass strands that would wick water in to the glassfibre to start osmosis.
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My 1970s Westerly - like many others - developed a similar but odd problem in which rashes of hemispherical pits about 3mm in diameter formed in dry area - cockpit seats in my case. Never underwater and rarely anywhere with standing water. I wonder if air bubbles explain that?
 
ghostlymoron said:
What do you mean by marine epoxy? I was planning to use Plastic Padding Marine Filler
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Expect that is epoxy based. check the recommendations. Think I would use something like International Watertite to skim and fair the whole area before painting.
 
JumbleDuck said:
My 1970s Westerly - like many others - developed a similar but odd problem in which rashes of hemispherical pits about 3mm in diameter formed in dry area - cockpit seats in my case. Never underwater and rarely anywhere with standing water. I wonder if air bubbles explain that?
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Very likely. It was due to them not letting the gel stand for the air bubbles to rise to the surface. The staff were more interested in getting on with the work. It only needed one man to use the gel too soon for a problem to occur later in time, so how many boats did he work on each week/month/year.

This fool has left these bubbles all over the mouldings, in my case patches underwater and a few in the top sides, but none on deck. As the mix was used up there would be less air bubbles, hence the patchiness. My 1981 Fulmar was produced at a time they could not build them fast enough due to a very full order book. As some mouldings do not have any problem it must come back to one or possibly two individuals out of the moulding team not working correctly. Whether that was down to bad management, poor quality control or ignorance, I doubt if we will ever know now.
 
JumbleDuck said:
My 1970s Westerly - like many others - developed a similar but odd problem in which rashes of hemispherical pits about 3mm in diameter formed in dry area - cockpit seats in my case. Never underwater and rarely anywhere with standing water. I wonder if air bubbles explain that?
Click to expand...

You will often see this on older boats, particularly if noticeable near curves like gunwales on toe rails, it's the result of thin Gel-coat in the original lay-up.

Getting the gel-coat into these areas is not easy with a pray gun and it's made worse when the chopper gun operator has the same problem.

The fix is major surgery, so just sand and fill as they appear.

Good luck and fair winds. :)
 
Quote Originally Posted by Davegriff View Post
Serious question. I see this stated a lot lately, but why is it not exactly? My boat is made of polyester resin as are most fillers and its waterproof, having been afloat 30 odd years!

Hi Dave,
Standard resins including polyester resin do not bond together very well when a new layer is applied to an older layer. The bonding is only mechanical. No real problem if the item is to be under compression like a padding plate.

An Epoxy resin will bond to Poly resin both mechanically and chemically, giving a much stronger bond.

Also note that underwater you also have a layer of gel-coat to protect the poly resin, are you planning to add this, because it's not required if an Epoxy resin is used as it far better at repelling moisture.

I Hope this helps.

Good luck and fair winds. :)
 
What I have seen on 3 diffeent boats now is small blisters totally dry inside and no smell and all were on parts of the hull above water where the hull had been covered by something. In one case a SSR No on the coachroof, a boat nameplate on the side and a boat name decal on the stern. As has been noted there are a myriad of reasons for gelcoat blisters apart from the dreaded O. Fill and fair and go sailing!
 
On further investigation it is obvious that the small blisters are just in the paint. So I've just sanded them down. All prepped and masked off now ready for paint tomoz when no doubt further blemishes will appear.
 
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