If not osmosis, then what is this??

Re: Why over-do the work ?

Yes, on the fine scale. I did check a number of boats on the yard for comparison. Note thogh that those were all boats that had been out of the water since last autumn. This is 60N in the Baltic where most people put the boat in May-June and lift her up again Sep-Oct. I will naturally repeat the measurements when I lift her again.

Yes. I will definitely put on a cheap primer on the exposed gelcoat and do two coats of gelshield next spring when she has dried out.

Remember that I have to do something anyway and it has been 10 years since last and then I lacked the stamina to do a really proper job of it /forums/images/graemlins/smile.gif

So sand down to get rid of all old primer, clean, dry, primer, AF.
 
<<especially for an old boat in cold Baltic waters>>

Actually as the Baltic is near fresh water ... low salinity - it can migrate into hull easier than saltwater ... given a chance.

Temp. of water doesn't really have that much difference on effect until you hit extemes. BUT note Baltic waters in Summer are warmer than UK / most areas in N.Europe ...

/forums/images/graemlins/smile.gif
 
I'd go with small_boat _champ, there's no point in trying to treat a small area. Even if it is osmosis, unless you are really planning on selling soon then who cares. Slap on some primer and antifould and get out there. Get a professional to advise you next year when shes out of the water, but any real osmosis treatement will probably be more than the boat is worth anyway.

You do know it'sa legal obligation to have your boat sjösätt before midsommer dont you :P
 
Trust me... Before I built the house, I usually had the boat in the water by May 1st... /forums/images/graemlins/smile.gif

And no. I never planned to treat just a small area, but I have sanded down approx 1 sq m to the gelcoat so I thought it best to put something on that before the AF.

Also, based on humidity readings and what I have read here, there is probably no need for any osmosis treatment, but preventive one. Ie primer, GelShield 200 or similar. I am not talking the full GelShield treatment with a million layers.
 
From the pictures you seem to have dimples not blisters. On the white areas (gelcoat I assume) there are definite hollows. Could you be suffering from the reverse of the tropical gelcoat problems. On older hand built hulls there are lots of voids (air bubbles). Mostly they are small <1mm.

With old gelcoat that has lost a lot of its elasticity the heat of the tropics can raise the pressure in the voids and cause domes to be formed on the surface. No Osmosis or chemical breakdown is evident.

Could you be seeing the opposite affect due to your hull drying out and the gelcoat sucking inwards.

If it was my boat I would get on and sail this year and think about having all the gelcoat removed in a few years time when life permits. If you want to treat for osmosis then you must remove the gelcoat on old boats. Epoxy on to carefully roughened/prepared/clean fibre glass the only way to do a lasting job. The gelcoat/glass interface must be removed but not too much of the glass.
 
"From the pictures you seem to have dimples not blisters. On the white areas (gelcoat I assume) there are definite hollows."

Nope. The gelcoat is ruby red. The white/yellowish stuff is old primer. When sanded, it looks almost like there had been tiny bubbles in the primer where it did not adhere to the gelcoat. Ie when sanding down on areas where the paint has never flaked, you first have a smooth white surface, then the pattern starts to appear as you sand down, removing more and more of the old primer.

There probably not much point in removing all gelcoat since the cost of that will exceed what the boat is worth anyway.

Your theory about bubbles might have some merit though.
 
This is sounding more like a paint adhesion problem than osmosis. As I said earlier, check out the boatyard owner's reasons for suspecting osmosis.

If he can't come up with anything convincing, forget about osmosis. As others have said, primer, antifoul, and go sailing. Your season sounds to be even shorter than ours, so don't waste it trying to solve problems that don't exist.
 
OK, Red = gelcoat, white = paint. Sorry too used to white gelcoat.

But that begs the question of the discolouration of the gelcoat? Dark reds and light reds.

If you sand further or scrape does it get darker? If that is the case then some of the dark red patches are quite large and should be investigated.

The small red patches in the white areas are probably due to tiny air voids expanding. Are there any patterns like lines? Or is uniform all over?

Only really worry if you find the nasty juices or odours?
 
"f he can't come up with anything convincing"

True. This guy has a tendency to be very "cathegorical", ie "this is the way it is", at the same time does he have a tremendous amount of experience. When he saw the bottom he said immediately that it was osmosis. When I showed him that there was no voids underneath and no smell, flud etc, he changed his mind and claimed the hull was saturated with moisture and it was seeping out. I guess the Tramex ruled that out as well. You are probably right that there is no problem other than paint adhesion.

"But that begs the question of the discolouration of the gelcoat? Dark reds and light reds."

The light is from the sanding. Now it is entirely possible that the surface is sligtly uneven and the orbital sander leaves small dimples untouched. I will try rubbing over with a piece of paper in my hand to see if there is any difference.

BTW. Thanks for all the input. I REALLY appreciate it!
 
I also did my hull with a ROS with a final 320 grit sand , maybe this is why the dimples appear and if done by hand we would have never seen the (problem) .
I read somewhere that gel coat doesnt come in differant colours so maybe the red you see is still paint ? Im not sure if i read it as someones opinion or if it was fact . Maybe someone could put me right on this ?
 
Baltic ...

Anders .... I am across the water in Ventspils Latvia ... so know well Baltic waters ...

I have a small 550 Kormoran as boat at my house on river, recently sold my 1/4 ton race boat out of Tallinn and also have a 25ft MS in UK ....

Back to Baltics ... warm summers - cold winters !! At moment it's glorious !!
 
Re: Baltic ...

"I read somewhere that gel coat doesnt come in differant colours so maybe the red you see is still paint ?"

Nope. It is pigmented gelcoat. I know this for a fact. Spent one summer working in a shop where we sold the stuff when I was a kid in the 70s.
 
Gelcoat ...

It was preferred in years gone by to not pigment gel-coat as it was believed to weaken it. But for many years pigmented gel-coat has been and is still available.

The boats that tended to not be pigmented were built using different resins etc. to your normal production boats ... specialist racers etc. Then they were spray painted in all the sponsor logo'd / designer colours etc.

A point that I should have made earlier ... a boat can be over-painted and suffer particles in the coating ... when its removed / sanded back - it gives appearance of minute bumps / lighter colour pips ... sometimes mistaken as in the gel-coat. It's actually where gel colour surface is affected.
 
Re: Gelcoat ...

Yes, another point is that many colours, especially darker ones used in the 70s did not age very well and aquired a "chalked" look after a while. That is why this boat (topsides) was painted over by the previous owner.
 
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