If not osmosis, then what is this??

AndersGu

AndersGu

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Hi!

I need input on this in case anyone has an idea?

When sanding down the bottom of my 30+ year old boat, (Did that last 10 years ago) the boatyard owner said that I had osmosis. Problem is that (see pictures below) it does not quite fit the bill, since.

- There are no blisters
- If you dig, and you have to dig hard, there is no fluid noticeable and no cavity underneath the gelcoat
- A Tramex Skipper shows a very low reading, 11-16% on the most sensitive scale, below the waterline. This is comparable to other boats on the yard.

The boat is over 30 years old as I said and has been sailed in the Baltic only. It has usually been in the water for 4-6 months per year.

What I have in mind is to put a layer of gelshield on the exposed gelcoat, lightly sand the rest and put on antifouling. Then launch and sail until end of August. Lift the boat. Sand down to the gelcoat all over and let it dry until next year and then do a proper gelshield job, perhaps 4 coats around midsummer.

Pictures
http://www.pedago.fi/dalton/diverse/dsc_0072.jpg
http://www.pedago.fi/dalton/diverse/dsc_0073.jpg
http://www.pedago.fi/dalton/diverse/dsc_0074.jpg
 
It sounds like you need to get another opinion from a surveyor. If it isn't Osmosis there seems little point in treating it for osmosis! Even less point in giving it a short term treatment to just last for the summer.
 
True... The total value of the boat is probably around 3000 euros anyway so there is not much point in pouring money over it, nevertheless am I quite fond of this 24 footer so I'd rather keep it.

Anyway, the reason why I did this was because the antifouling and primer below had flaked in places. What I am aiming for is to do something that will last another 10 years /forums/images/graemlins/smile.gif
 
I would ask the boatyard owner what makes him think you have osmosis - the obvious pointers seem to be absent.

If he does make a reasonable case, then you have to ask yourself: Do I care? Many folk take the view that osmosis never sank a boat, and it only becomes a problem when you come to sell.

If you decide that you do care, seek a second opinion from a reputable surveyor, and discuss possible courses of action with him. Half measures are a waste of time & money, and can be counter-productive if you trap excessive levels of moisture in the hull.

Don't see the point of small patches of Gelshield now. If you want to put something on the bare gelcoat, use an antifouling primer.
 
Can you give me any examples of antifouling primers?

When I did the full works on the bottom last, I used a 2-component primer from Hempel, followed by Hempel Mille. I did not sand it down completely at that time though.

Is an antifouling primer superior to Gelshield in your opinion?
 
If there is low moisture reading and no blisters then I don't see that you have osmosis.

Only caveat I could suggest is to check Tramex on a few boats just to make sure it does detect moisture.

If thats ok forget it -carry on sailing.

The world is well supplied with pessimists.
 
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Can you give me any examples of antifouling primers?

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OK, International Interprotect, or VC Tar if you are using VC Systems. Blakes - Underwater Primer


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When I did the full works on the bottom last, I used a 2-component primer from Hempel, followed by Hempel Mille. I did not sand it down completely at that time though.

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Dont know Hempel products, so can't comment


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Is an antifouling primer superior to Gelshield in your opinion?

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They do different things. Gelshield is intended to act as a moisture barrier, whereas a primer is intended to attach subsequent coats to the substrate.

Having said that, I have just checked International's product guide, and see that they recommend a single coat of Gelshield as an alternative to Interprotect.

If the initial problem was flaking paint, that suggests a compatability or contamination problem. Thoroughly clean the gelcoat with something like Yachtline Super Cleaner before sanding.
 
Bergman!
Yes, I did check the Tramex on several boats on the yard and got similar readings, ie 11-16%

Shanty!
It can very well be that this boat had this problem from manufacture as I remembered seeing what appeared to be "bubbles" in the original primer when I sanded it down, much like what can be seen in the pictures where you see the yellowish primer on the ruby gelcoat. No idea if that super cleaner is available up here. Perhaps Acetone works equally well?
 
I asked the same questions 6 months ago when i had my boat lifted and found the same thing .
cuchilo004.jpg

After asking around the best answer i got was from a boat builder that said uneaven fibreglassing .
He then went on to say that a 30 year old boat is bound to have osmosis anyway /forums/images/graemlins/frown.gif
 
No there was no actual bubbles they just looked like it because some places sanded off before others .
After sanding back i washed off the dust and used three coats of Blakes underwater primer and three to four coats of Blakes antifouling . Oh and a hard racing antifoul for the waterline .
 
OK. Interesting. Your approach sounds pretty much like what I have planned, ie to sand down, perhaps clean with acetone after a soap wash, then primer or gelshield 200, then antifouling.

Did you ever check moisture content on your hull?
 
The Super Cleaner product is basically a strong detergent. You are trying to remove any traces of oil / grease, and any mould release wax still left after 30 years.

Acetone should do the job, but do the cleaning before you sand, rather than after. If you sand before cleaning, you will embed any contaminants in the surface of the gelcoat, and make them harder to remove.
 
Unless you are definite that the hull is very dry then I wouldnt use gelshield or any other epoxy. This will trap any moisture in.

use an ordinary one pot primer recommended by your antifoul manufacturer.
 
"Acetone should do the job, but do the cleaning before you sand, rather than after."

Problem is that I need to get rid off all old primer before I can get to the gelcoat. The antifouling was removed by waterblasting.

"Unless you are definite that the hull is very dry then I wouldnt use gelshield or any other epoxy. This will trap any moisture in."

That is definitely true, but the tramex does indicate a dry hull, below what is recommended as a maximum for Gelshield. I guess I will decide finally in a year, after sanding down this september, washing down a coupld of times and letting it dry under a tarp until June 2007 /forums/images/graemlins/smile.gif
 
If the hull is dry enough to epoxy now why wait a year?Personally I would get some paint and antifoul on and carry on enjoying the boat.
 
Sorry. It is because it will take me too long to get the entire hull sanded down to the gelcoat now. The entire project stalled when the yard owner said I had osmosis.

So I will do what you say essentially. Cover the approx 1 sq m I have sanded down with primer. Lightly sand the rest and go sailing until end of August. Then do a proper strip job, let dry until next spring and do a proper paint job, just to get rid of excessive flaking.
 
Never seen that before, however it looks like oxidation, could be particular to the brand that you applied previously. I would wet sand and antifoul as normal. Possibly the powder is an alkalai oxidation from a metal compound in the previous coating. This is definately not osmosis or anything to be to concerned about.
 
Why over-do the work ?

You say the Tramex shows 11 - 16% on scale ... I take it that is oin the fine scale ? It is pretty dry if you are getting readings like that. You also say that other boats in the yard had similar readings ? Unusual - I have to remark. I have Tramex and Protimeter as tools of trade and I would expect most boats in a yard to give high readings - many to go off-scale ! with a Tramex.

Applying a small amount of Gelshield - If it was my boat ? I would not bother. Unless you have an all-over proper applied coat - patches are a waste of time and money.
Your boat is 30 yr old and if the Tramex is working correctly - is dryer than most boats of her age .... I would coat up with a good primer such as International or Jotun AF primer and then give it its couple of AF coats ... Decide later if its worth the effort to Gelshield .... which means later when she's lifted out before winter etc.

IMHO of course though ...
 
I'm with you on this...coating an old hull with epoxy is a pointless task unless its unavoidable due to advanced osmosis (which this is obviously not).

When I bought my Halberdier it had a similar problem which had put off other buyers, who suspected osmosis....but what in fact turned out to be blistering under a coat of polyurethane paint which had been applied to the hull when new in 1970.

I slurry blasted off all the old a/f and the polyurethane paint in the areas where it was blistered, applied underwater primer re antifouled and went sailing. That was 6 years ago..

This osmosis thing as mentioned ealier is all a bit overstated especially for an old boat in cold Baltic waters.

Nick
 
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