Osmosis?

[JumbleDuck]

I spent a cheerful hour or two poking around under my wee Hunter today. This is what I found.

There are hundreds and hundreds of blisters between the paint and the gelcoat. These are typically 3 - 6mm across and perhaps 1 - 3 mm high. When burst a clear fluid smelling strongly of acetone comes out. Underneath the paint the gelcoat is completely unblistered, but under perhaps half of the paint blisters there is a small hemispherical pit in the gel coat, typically 1mm deep. It's not impossible that there are pinholes under the blisters which don't have obvious pits.

The boat is about 40 years old and has done three seasons on a freshwater loch after 20 years ashore.

The only alternative to osmosis I can imagine is some strange reaction between the Trilux I put on when I recommisioned her and the old paint underneath, but the fluid filling to the blisters makes that seem unlikely. If it is osmosis, it seems to be finding it easier to squirt out through pinholes than to push off the gelcoat, which I find reassuring.

My plan is to strip it down to the gel coat, fill all the pits and repaint. It wouldn;t be worth a full osmosis treatment even if I could afford it and was worried enough to contemplate it.

Opinions welcome.

 

[CAPTAIN FANTASTIC]

This is a winter job; remove all paint down to gelcoat and apply 2-3 coats of Jotun Penguard hb two part epoxy, then two coats of hard racing antifouling and then normal antifouling; job done in 4 days and good for another 40 years.

 

[JumbleDuck]

This is a winter job; remove all paint down to gelcoat and apply 2-3 coats of Jotun Penguard hb two part epoxy, then two coats of hard racing antifouling and then normal antifouling; job done in 4 days and good for another 40 years.

Thanks. Alas it is not winter. Since the boat is going to live in fresh water for the foreseeable future, I am not going to bother with antifouling this time: a couple of barrier layers and then ... what? See new thread ...

 

[TradewindSailor]

Either the paint or the gel coat hasn't cured properly. The chemicals residing close to the surface have sucked in water and created the blister that now contains a concentration of dissolved chemicals that are breaking down the gelcoat.

If you remove the blisters and paint, and coat with an epoxy barrier coat it should be fine.

Have you ever used paint stripper on this area of hull? ...... It can break down the gel coat.

 

[JumbleDuck]

Either the paint or the gel coat hasn't cured properly. The chemicals residing close to the surface have sucked in water and created the blister that now contains a concentration of dissolved chemicals that are breaking down the gelcoat.

If you remove the blisters and paint, and coat with an epoxy barrier coat it should be fine.

Many thanks.

Have you ever used paint stripper on this area of hull? ...... It can break down the gel coat.

No ... or rather, not yet. I did the bit above the waterline with Removall when I did the boat up, and that bit seems fine. I'll be using the same on the underwater portions. It's a non-caustic stripper, explicitly sold for use on GRP.

 

[oldsaltoz]

This could be pin holes in the paint, a few days on the hard and most of them should disappear, particularly the ones on or just above the waterline.

This is not the big 'O'.

It's more like poor preparation/application when painted.

Good luck and fair winds.

Good luck and fair winds.

 

[JumbleDuck]

This could be pin holes in the paint, a few days on the hard and most of them should disappear, particularly the ones on or just above the waterline.

This is not the big 'O'.

It's more like poor preparation/application when painted.

Thanks. She has been shore since October, and the blisters appeared while she was afloat (I know, I know, I should have checked earlier, but have you seen a Scottish winter?). There are no blisters anywhere above the waterline.

 

[boatmike]

After 40 years if that's all you have then your plan is more than adequate. There are many possibilities regarding the cause but who cares? You have already decided it's not worth a complete gel coat replacement and you are right.

 

[JumbleDuck]

After 40 years if that's all you have then your plan is more than adequate. There are many possibilities regarding the cause but who cares? You have already decided it's not worth a complete gel coat replacement and you are right.

Many thanks. Have you - or has anyone - any bright ideas for filling the myriad indentations on the hull?

 

[Blueboatman]

Arguably a bodge is the worst thing to do..Patching traps moisture in, under pressure over time..

Do nowt perhaps? You canna see it once its wet, after all

Then bung it in the garage over winter and remove the old paint, the lovely gelcoat, dry it all out properly, epoxy hibuild paint it, fair it, more epoxy...yeah right....Just sail and enjoy!

I thort you was on yer way to Ireland, not looking for problems?

 

[JumbleDuck]

Arguably a bodge is the worst thing to do..Patching traps moisture in, under pressure over time..

Do nowt perhaps? You canna see it once its wet, after all

I don't think "doing nothing" is an option, as she looks pretty shabby underneath, all oozing lumps. Think "teenage boy". However, "not doing much" is certainly an option, and one I like. If this is some sort of paint reaction, which seems less likely, then taking it all off and repainting is clearly a good plan. But, if it is something osmotic, would I make things worse by putting on some epoxy barrier? If I paint it normally and the pustules reappear, a few hours and a bucket of paintstripper gets me back to where I am....

I thort you was on yer way to Ireland, not looking for problems?

That's the potential summer trip in the bigger boat, which lives 100 miles and a ferry trip from home. The Hunter lives ten miles away and is my sod-it-work-can-wait-till-the-evening option for nice days. Sailing on a smaller scale, as it were.

 

[Elessar]

I spent a cheerful hour or two poking around under my wee Hunter today. This is what I found.

There are hundreds and hundreds of blisters between the paint and the gelcoat. These are typically 3 - 6mm across and perhaps 1 - 3 mm high. When burst a clear fluid smelling strongly of acetone comes out. Underneath the paint the gelcoat is completely unblistered, but under perhaps half of the paint blisters there is a small hemispherical pit in the gel coat, typically 1mm deep. It's not impossible that there are pinholes under the blisters which don't have obvious pits.

The boat is about 40 years old and has done three seasons on a freshwater loch after 20 years ashore.

The only alternative to osmosis I can imagine is some strange reaction between the Trilux I put on when I recommisioned her and the old paint underneath, but the fluid filling to the blisters makes that seem unlikely. If it is osmosis, it seems to be finding it easier to squirt out through pinholes than to push off the gelcoat, which I find reassuring.

My plan is to strip it down to the gel coat, fill all the pits and repaint. It wouldn;t be worth a full osmosis treatment even if I could afford it and was worried enough to contemplate it.

Opinions welcome.

OK stop worrying.

Yes it is osmosis - ie "a process by which molecules of a solvent tend to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one."

But not the bad thing as we know it. Osmosis is a strange word with lots of meanings like "video". "Put the video in the video and video Eastenders."

The key is that the blisters are between the paint and the gelcoat. Osmosis as we know it is a blister between the gelcoat and the laminate, in this case the gelcoat is the semi permeable layer.

In your case it is probably caused by poor keying or a reaction as you said, ans the water has gone through the paint - antifoul is porous anyway.

Get all the paint off. Abrade with 80-120 grit. Prime. Antifoul. Sail. (there was not the word "worry" in that list)

IF it were osmosis as we know it, it is possible in a very advanced state to split the gelcoat instead of bubbling. However it seems inconceivable to me that every bubble would split. If you have NO bubbles between the gelcoat and the laminate as you have described I'm confident in my diagnosis.

 

[JumbleDuck]

IF it were osmosis as we know it, it is possible in a very advanced state to split the gelcoat instead of bubbling. However it seems inconceivable to me that every bubble would split. If you have NO bubbles between the gelcoat and the laminate as you have described I'm confident in my diagnosis.

Many thanks. If it hadn't been for the acetone smell of the bubbles I'd just have said "Meh, paint reaction". Strip, paint and sail sounds like my sort of solution.

 

[Blueboatman]

There you go, sorted.( Just add labour ahem)...Strip off back to the gelcoat then have a looksee at the hull moulding.

 

[JumbleDuck]

There you go, sorted.( Just add labour ahem)...Strip off back to the gelcoat then have a looksee at the hull moulding.

There are a few places where I have been able to prise off quite large patches of paint, and, small pocks apart, the gelcoat looks fine.

 

[Elessar]

There are a few places where I have been able to prise off quite large patches of paint, and, small pocks apart, the gelcoat looks fine.

Excellent