Omosis, or what else.?!?!

[wazza]

I'm scraping the hull of my boat. She's a Najad 343 from 1981 and she appears to have hundreds of tiny spots. Blisters, for me are larger and can be popped, these cannot.
I've run my hand over them and they certainly aren't smooth but on the other hand they aren't, again what I'd call blisters as I've tried to cut them open and it doesn't feel like I pop them...?!
Any thoughts, I understand it's difficult to give a concrete opinion and I can't seem to upload a picture that I took on my phone onto this thread... Amish possibly..
If it is osmosis can I scrape, sand the epoxy primer and all will be well or.?!?!

 

[David2452]

Just bought a boat for rental with similar appearance, having the boy dig out the major blisters then after A/F removal will get it gone over with a belt sander to remove the mini blisters as a belt and braces then a couple of months drying and a couple of coats of epoxy after filling. 40 feet x 12 river cruiser is going to take a lot of epoxy

 

[Concerto]

When I was looking at buying my Fulmar, it had something very similar. An expert is osmosis diagnosed these very small blisters were due to air bubbles within in the gel coat. The gel coat had not been stirred, but whisked. Then it was applied to the mould and would make a good coverage of the mould, but over time water has caused the bubbles to expand. This means the expansion took the path of least resistance and caused small bumps on the surface of the gel coat. It does not mean the boat has osmosis, but some simple remedial action should be taken. Sand the hull smooth and the fill any small holes with an epoxy based filler and sand smooth. The apply 3 coats of GelShield or similar, followed by 2 coats of hard antifouling and then 2 of soft. I was told not to copper coat the hull as some areas in the future may require further remdial action and too much GelShield/epoxy makes this more difficult.

 

[William_H]

These blisters may be simply blisters in the old build up of anti fouling paint. Need to sand or grind down to see how deep the blisters are. good luck olewill

 

[charles_reed]

I'm scraping the hull of my boat. She's a Najad 343 from 1981 and she appears to have hundreds of tiny spots. Blisters, for me are larger and can be popped, these cannot.
I've run my hand over them and they certainly aren't smooth but on the other hand they aren't, again what I'd call blisters as I've tried to cut them open and it doesn't feel like I pop them...?!
Any thoughts, I understand it's difficult to give a concrete opinion and I can't seem to upload a picture that I took on my phone onto this thread... Amish possibly..
If it is osmosis can I scrape, sand the epoxy primer and all will be well or.?!?!

Had a similar experience in Malta, small bubbles, 2-3mm in diameter and 0.2mm high, around the waterline area. - the (very experienced) yard GRP specialist said it was small voids in the gelcoat, filling with water on the sunward side of the boat.
Faired down, 5 coats of epoxy, two of hard antifouling and two of eroding. 10 years later, on 2nd insurance survey, no evidence of water take-up.
It's an idea to have a very different colour hard to subsequent coats of eroding antifouling, you can then see the areas of fastest wear. As a result I have one coat to last 6 months in water and about 1.2 - 1.5K nautical miles of sailing.

 

[wazza]

Thanks all for your replies.. So it would appear that it seems to be osmosis, in an early stage.
So I will continue to scrape, then sand. What grit are we talking about. Should it be aggressive 80.?
Then how long should I leave the hull before epoxy primer goes on.? The temp here in Sweden will fall rapidly when the winter comes (Oct/Nov) and ideally I'd like to start the epoxy process before winter sets in..

 

[CFarr]

If it is osmosis, and I'm not convinced by the description, then you won't be able to get it done before winter.
Osmosis is fluid trapped within the laminate that needs exposing in order to dry out - which can take months.
If you re coat before the hull has dried out enough then you've simply wasted your time as the blisters will return.

You need to determine if your problem is osmosis.
If you scrape off a bubble/blister and there is no vinegar smelling dark fluid then the chances are you're in the clear.
Have you had the hull measured for moisture?
It will be quite high if it's osmosis.

 

[wazza]

If it is osmosis, and I'm not convinced by the description, then you won't be able to get it done before winter.
Osmosis is fluid trapped within the laminate that needs exposing in order to dry out - which can take months.
If you re coat before the hull has dried out enough then you've simply wasted your time as the blisters will return.

You need to determine if your problem is osmosis.
If you scrape off a bubble/blister and there is no vinegar smelling dark fluid then the chances are you're in the clear.
Have you had the hull measured for moisture?
It will be quite high if it's osmosis.

Firstly thank you for your reply.
The (thousands of them) tiny blisters are so small that when I cut them nothing seems to come out or smell.
What do you think it could be.? And incidentally they seem to be just on one side, although I haven't scraped so much on that side...

 

[JumbleDuck]

I had something similar on the bottom of my Hunter 490. There were blisters, but only under the paint, filled with acetone-smelling fluid which had evidently come out of tiny indentations (1 or 2mm across and deep) in the fibreglass. Apart from these the fibreglass was completely smooth. It may have been some sort of osmosis, but I gathered that it can also be a result of a reaction between two-pack paint and gel coat, and from the effort it took to strip I suspect there was two pack there.

A complete paint strip and nine coats of gelshield later, she looks quite smart again, though since she was only in the water for three days this year I haven't really tested things.

 

[Bobc]

They probably are osmotic blisters, but my advice would simply be to ignore them.

If you grind the off, you'll need to then let them completely dry out (which won't happen between now and April), and then you fill them and re-paint only to find more new blisters appearing next season.

The ONLY thing worth doing is a complete scrape, dry-out, and epoxy, and it's not worth doing it unless you're selling the boat.

 

[wazza]

They probably are osmotic blisters, but my advice would simply be to ignore them.

If you grind the off, you'll need to then let them completely dry out (which won't happen between now and April), and then you fill them and re-paint only to find more new blisters appearing next season.

The ONLY thing worth doing is a complete scrape, dry-out, and epoxy, and it's not worth doing it unless you're selling the boat.

Isn't that a little like sticking one's head in the sand.??!? Surely if I just close my eyes and pretend they aren't there it'll just be a worse problem eventually.???

 

[CFarr]

They probably are osmotic blisters, but my advice would simply be to ignore them.

If you grind the off, you'll need to then let them completely dry out (which won't happen between now and April), and then you fill them and re-paint only to find more new blisters appearing next season.

The ONLY thing worth doing is a complete scrape, dry-out, and epoxy, and it's not worth doing it unless you're selling the boat.

+1
That's exactly what I did and had 18 months of frustration whilst I impatiently waited for her to dry out.
Get someone to take moisture readings of the hull - if it's osmosis she'll be up in the 20+ range - mine was anyway.

If you do decide to get her treated it's like bobc says, gelcoat peel and wait, and wait, and wait.
Mine even needed an extra peel of the laminate before she started to dry and that was with the occasional use of vacuum heat pads

 

[Rivers & creeks]

They don't sound like osmosis. Much more likely as has been said to be burst or un-burst air bubbles in the gel coat. They're the right era for it as well. Tiny osmosis blisters are commonly found on and just above the waterline even though they are probably nowhere else. It is not only pointless but harmful to epoxy coat now, otherwise you're locking in the moisture that would normally leave during the winter, so if you're determined to do it - leave her out until next spring. Or just as good carry on regardless, fill and fair where necessary. Millions is spent supporting the osmosis industry in 'preventative' coatings and even worse osmosis 'cure'.

 

[Bobc]

Isn't that a little like sticking one's head in the sand.??!? Surely if I just close my eyes and pretend they aren't there it'll just be a worse problem eventually.???

To a degree, yes, unless you're prepared to spend the think-end of £10k and 9 months getting it done properly. Mucking about with it is just a waste of time, money, and effort.

 

[Concerto]

Thanks all for your replies.. So it would appear that it seems to be osmosis, in an early stage.
So I will continue to scrape, then sand. What grit are we talking about. Should it be aggressive 80.?
Then how long should I leave the hull before epoxy primer goes on.? The temp here in Sweden will fall rapidly when the winter comes (Oct/Nov) and ideally I'd like to start the epoxy process before winter sets in..

Reread my post #3. It is not osmosis, but air bubbles in the gel coat. If you can still see gel coat at the bottom of the burst bubble, then the gel coat has not allowed water penetration of the glassfibre.

Continue to scrape and sand. 80 grit is not too agressive as if you are to apply GelShield epoxy coating (or similar), you will need to remove all traces of old paint and leave a good key for the GelShield.Boats of your age usually had a thick layer of gel coat measured in millimetres, so there is plenty to work with.

If no water has penetrated the glassfibre, then you can start the epoxy coating immediately as the internal structure or the glassfibre will not be wet.

So get sanding and filling, then paint.

 

[wazza]

Reread my post #3. It is not osmosis, but air bubbles in the gel coat. If you can still see gel coat at the bottom of the burst bubble, then the gel coat has not allowed water penetration of the glassfibre.

Continue to scrape and sand. 80 grit is not too agressive as if you are to apply GelShield epoxy coating (or similar), you will need to remove all traces of old paint and leave a good key for the GelShield.Boats of your age usually had a thick layer of gel coat measured in millimetres, so there is plenty to work with.

If no water has penetrated the glassfibre, then you can start the epoxy coating immediately as the internal structure or the glassfibre will not be wet.

So get sanding and filling, then paint.

Had a guy who has works with gel coat look at it yesterday and he'd never seen it before. I've scraped away at the blisters and there seems to be no sign of weeping so I believe your assumption is correct and they're air bubbles...
But it is a s##ty job though...