Using an Epoxy Barrier Coat over Gelcoat

keensailor

keensailor

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In about 4 months we need to repair about 8 blisters ranging in size between 10mm and 35mm. After the repair I was planning to coat the whole hull with epoxy (International Interprotect). The boat is only going to being out of the water for a week during the whole process. However, after reading various forums I'm not sure it is a good idea to epoxy over the gelcoat in the risk of "trapping" the moisture in and ending up with with blisters everywhere ? Another guy in our club had this happen to him.
My boat is 29 years old and has never had blisters repaired. Am I right in thinking if I get these 8 done it may take another 29 years for a few others to develop (without an epoxy barrier coat). If so I am better off not using epoxy.
 
I have a similar question: my mate has bought a catamaran and he has had the hull 'soda-blasted.' It is back to gel coat, abraded, with some areas where the mat is showing. The boat has been ashore over the winter and the surveyor has said she is reasonably dry, recommending multiple fresh water wash downs and then the hull left to dry in the wind and sun before coating. I understand that the areas of open mat will require filling with epoxy and I think I am happy with doing that bit, but any advice on the whole task would be most gratefully received. I am thinking a few coats of epoxy as described (International Interprotect?) then the silver antifoul primer before top coating with antifoul? Am I on the right track here please?

I must add that for a 1979 boat, there is virtually no sign of any osmosis blistering, only one or two blemishes (approx 35mm) which could have been something else.
Many thanks.
Steve
 
keensailor said:
In about 4 months we need to repair about 8 blisters ranging in size between 10mm and 35mm. After the repair I was planning to coat the whole hull with epoxy (International Interprotect). The boat is only going to being out of the water for a week during the whole process. However, after reading various forums I'm not sure it is a good idea to epoxy over the gelcoat in the risk of "trapping" the moisture in and ending up with with blisters everywhere ? Another guy in our club had this happen to him.
My boat is 29 years old and has never had blisters repaired. Am I right in thinking if I get these 8 done it may take another 29 years for a few others to develop (without an epoxy barrier coat). If so I am better off not using epoxy.
Click to expand...

Watch this YouTube episode by Free Range Sailing, and maybe the next couple for the whole bit on osmosis repair:

https://m.youtube.com/watch?v=U2td_GkwX4w

The whole series is a good watch, with lots of useful tips. I look forward to each new release.
 
WightMistress said:
I have a similar question: my mate has bought a catamaran and he has had the hull 'soda-blasted.' It is back to gel coat, abraded, with some areas where the mat is showing. The boat has been ashore over the winter and the surveyor has said she is reasonably dry, recommending multiple fresh water wash downs and then the hull left to dry in the wind and sun before coating. I understand that the areas of open mat will require filling with epoxy and I think I am happy with doing that bit, but any advice on the whole task would be most gratefully received. I am thinking a few coats of epoxy as described (International Interprotect?) then the silver antifoul primer before top coating with antifoul? Am I on the right track here please?

I must add that for a 1979 boat, there is virtually no sign of any osmosis blistering, only one or two blemishes (approx 35mm) which could have been something else.
Many thanks.
Steve
Click to expand...

I had some blisters on my 43 year old hull 2 seasons ago. I merely ground the area, washed with clean water several times and let it dry out. Then just overcoated the existing grp mat with new resin, then Interprotect followed by Primocon, and then antifoul paint. Job done. No need to grind too much away, just sufficient to remove blister and around.
 
Thanks for that Appledore.
My main issue is that after the 'soda-blasting' which makes the existing gel coat look very rough, I am wondering if even the undamaged gelcoat surface should have some kind of treatment before the boat goes anywhere near the water. I am happy with the repairs of the localised bits.
 
I would not overcoat in epoxy. You already know the reasons, that epoxy can trap pressure from the molecules that would normally pass through the hull and cause blisters. Perhaps the reason you only have a few blisters is because the hull does not have epoxy over it. The reasons for so called osmosis showing can be down to the position of the poor quality resin mix in the original layup: deep in the matrix, near the surface inside, near the surface outside, matrix thickness, quality of the surrounding resin mix. In other words a lottery. There is a claim made in one book that the transfer of the water molecules through the hull is a good thing. I think, as others have said, fix what you can but don't do anymore than that. Anyway, getting the epoxy to stick is a big job in itself, cleanliness and surface preparation being vital for a successful bond. The link to the book I elude to is on another computer, later today, I'll post the link as an edit to this thread. My advice is an an amateur, owner of an old boat, who has researched this using Google, hence, may not be valid.
 
Thank you very much for the information, very much appreciated. I am concerned that after the soda blasting, something will need to be done to the hull to protect it but I am at a loss as to what. I look forward to hearing from you with the link.
Thank you again
Regards
Steve
 
If you are wanting to epoxy coat the hull, being out of the water for just a week is too short a period of time to allow any drying of the hull. My boat had been out of the water for 18 months before I bought her and the moisture reading were only just acceptable for epoxy coating. The surveyor also advised only 3 coats. If any water was trapped in the hull, it could blow the epoxy and make it more difficult to fix.

My advice would be to leave the boat out of the water for several months. Lightly sand the rough gel coat and then apply 3 coats of epoxy, 1 coat of primer and 2 coats of hard antifouling. On my boat I then applied 2 coats of self polishing antifouling. As the hard antifouling is a contrasting colour to the self polishing, I can visibly see when it need re-antifouling.
 
My own sense is

That the one week out of the water, defines your likely coating strategy, this time round. Having seen all the suggestions for International product, might I suggest Jotun product be considered. We had a troublesome resin coating under the original 1963/4 top coat. Sand blasting the entire hull, has left us with the opportunity to epoxy the whole hull.

Sandblasting exposed some deep craters and channels in the topsides. Hence, a major filling and fairing exercise still underway. Jotun Penguard used with a variety of filler mixes has been very pleasing. One person applying it and me mixing up the pots, was a pretty efficient routine. Though I might have kept two application persons working, I was not fully occupied mixing for one!

I am delighted and look forward to finishing. Happy to answer and questions or send pictures, not clever enough to add pictures to my posts!
 
I am humbled by the number of helpful replies I have had, thank you very much. The boat has been out of the water since Fri 2 November, and we expect to start on preparing and coating the hull at the end of March. The soda blasting has appeared to prepare and rough up the gelcoat beautifully, and loosen out some bits, showing mat, so some filling obviously required. The surveyor has told us the hull is drying nicely in the wind here in Gosport so we are hoping to coat the hull in a month or so's time (4 month's on the ground.) Thank you again for the advice, most helpful.. Regards, Steve
 
We were in a similar situation two years ago with our now 40 year old boat, though our blisters were only 2 - 3 mm diameter. Having removed all the old anti-foul and considering our next step, I phoned the technical guru at Gurit UK on the Isle of Wight (his name is Martin and he is indeed a guru) who, in a nutshell, said leave the bottom bare for as long as possible over the winter then prime, anti-foul, go sailing and enjoy the boat. Don't worry about the blisters and don't bother with epoxy, you'll be wasting your time and money and be sure to haul out every winter so as not to exacerbate the problem. This is paraphrasing heavily as the conversation was a couple of years ago and so isn't a direct quote!

He has a wealth of knowledge and expertise that he always seems happy to share, give him a call.

Incidentally, a lot of folk rave about WEST Systems epoxy and I agree it is good stuff but I prefer the Gurit range of products as I've had better results with them. No connection other than a happy customer.
 
Thank you Matt,
I really appreciate your response and am taking all this information with huge gratitude. My main concern is how far the soda blasting went and whether the gelcoat really needs a bit of protection before the antifoul primer and top coat goes one. Clearly we have some epoxy repair work to do; as for whether we do an all over coat of epoxy as well, I am still cogitating on this. I will talk to the Gurit guru here on the island (we live in Island Harbour, just up from the Folly Inn on the Medina) and maybe show him some photo's of the hull (which is in Gosport.) We will do the very best for our friend with this, even if it means Option One - do nothing. Thank you again, we really appreciate it.
Regards

Steve
 
You're welcome Steve.

Having re-read the thread, I think Appledore's advice is very sensible and most probably what I'd do, especially since it's the same as the advice given to us by Gurit. :)
 
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