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29-05-2022, 09:51
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#1
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Registered User
Join Date: Aug 2019
Posts: 205
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Once and for all, does the gel coat under the antifoul keep the water out or not???
I cant get a straight answer anywhere...
Yes, no, depends...
its a 30yo gel coat. Its probably polyester so does it absorb water over time?
Does antifoul let water through it
Is gel coat Impermeable?
impermeable surface means any surface which does not permit fluids to pass through or penetrate its pores
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29-05-2022, 09:56
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#2
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Registered User
Join Date: Jun 2018
Boat: Voyage 430
Posts: 401
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
I'm not an expert, so take this with a grain of salt. It is just one data point.
I owned a fiberglass boat that was gelcoated below the waterline and then had bottom paint put over that. Blisters developed on the bottom between the gelcoat and the fiberglass.
The boat was built in 1972 and I am unsure of the resin used in the gelcoat.
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29-05-2022, 10:07
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#3
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Nearly an old salt
Join Date: Jun 2009
Location: Lefkas Marina ,Greece
Boat: Bavaria 36
Posts: 22,801
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
Gel coat , ie polyester is not waterproof , it allows a certain amount to seep into the laminate.
__________________
Interested in smart boat technology, networking and all things tech
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29-05-2022, 10:08
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#4
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Registered User
Join Date: Aug 2019
Posts: 205
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
There are no blisters. The boat was built in mid 90s. It was a top of the line boat so im sure they used quality products. Jeanneau. Im just not sure if its polyester or epoxy gell coat. I assume polyester.
Is there anyway to determine what the gel coat is made of underneath the old antifoul, and how impermeable it is?
But isnt antifoul impermeable?
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29-05-2022, 10:09
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#5
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Registered User
Join Date: May 2015
Location: Muskegon, Mi
Boat: Columbia 36
Posts: 1,298
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
Quote:
Originally Posted by ThereAndBack
I'm not an expert, so take this with a grain of salt. It is just one data point.
I owned a fiberglass boat that was gelcoated below the waterline and then had bottom paint put over that. Blisters developed on the bottom between the gelcoat and the fiberglass.
The boat was built in 1972 and I am unsure of the resin used in the gelcoat.
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That boat was probably better quality resin than used in the late '70's and '80's. They cheaped it up after the oil crisis of 1973 and vinylester hadn't come along yet. Polyester IS permeable, vinylester less so and epoxy less than that. Very few production boats use epoxy, that's why aftermarket epoxy barrier coats are popular. And most antifouling is also permeable, that's how it works. The copper leaches out over time and kills or repels growth.
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29-05-2022, 10:11
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#6
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Registered User
Join Date: Oct 2007
Location: Lakewood Ranch, FLORIDA
Boat: Alden 50, Sarasota, Florida
Posts: 3,645
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
There is no vagaries here. Don’t know why you didn’t find a definitive answer.
Gelcoat is water -resistant, not waterproof. And anti fouling bottom paint is intended as neither.
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29-05-2022, 10:21
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#7
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Registered User
Join Date: Aug 2019
Posts: 205
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
Osmosis is a process whereby an imbalance of fear across a knowledge barrier causes money to flow from the high fear to the low fear side. A barrier coat of information can be used to block the flow and end the problem.
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29-05-2022, 10:31
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#8
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Registered User
Join Date: May 2016
Location: Denmark
Boat: Nordship 808
Posts: 324
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
Quote:
Originally Posted by GreenHeaven
Osmosis is a process whereby an imbalance of fear across a knowledge barrier causes money to flow from the high fear to the low fear side. A barrier coat of information can be used to block the flow and end the problem.
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Best post I've read in a long time
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29-05-2022, 10:31
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#9
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Registered User
Join Date: Dec 2008
Location: Noank, Ct. USA
Boat: Cape Dory 31
Posts: 3,280
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
Yes.
Yes.
No.
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29-05-2022, 11:00
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#10
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Registered User
Join Date: Dec 2014
Location: On a sphere in a planetary system
Boat: 1977 Bristol 29.9 Hull #17
Posts: 730
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
Quote:
Originally Posted by GreenHeaven
Osmosis is a process whereby an imbalance of fear across a knowledge barrier causes money to flow from the high fear to the low fear side. A barrier coat of information can be used to block the flow and end the problem.
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This right here^^^
We did a barrier coat on our 1977 Bristol, we had no blisters when we scraped it down to the Gel coat, it was a fair bit of work, but if “ waterproofing” your hull is what you are after, an epoxy barrier coat is your remedy,
Fair winds,
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29-05-2022, 14:01
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#11
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Hull Diver

Join Date: Oct 2006
Location: Under a boat, in a marina, in the San Francisco Bay
Posts: 5,483
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
Quote:
Originally Posted by S/V Illusion
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 In point of fact, anti fouling paint works by letting water penetrate its matrix.
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29-05-2022, 18:03
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#12
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Registered User
Join Date: Feb 2020
Location: Port adelaide south australia
Boat: Cheoy lee perry 48
Posts: 783
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
The answer is no gelcoat is water permeable under water pressure ie afloat not getting wet on the hard ,anti fouling paint is water permeable as stated that’s how it works ,epoxy on the outher hand is about as close to water proof as you will get on a grp built boat ,the purer the better ,at least that’s how it works in Australia .⛵️⚓️
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29-05-2022, 18:08
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#13
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Moderator
Join Date: Sep 2014
Location: Channel Islands, CA
Boat: 1962 Columbia 29 MK 1 #37
Posts: 15,298
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
Once and for all, no.
If there are no blisters, and you have no unusual leaks, there is little to cause concern.
I have a 60 year old boat with a very permeable gel coat and no blisters and various layers of bottom paint. No problems.
__________________
DL
Pythagoras
1962 Columbia 29 MKI #37
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29-05-2022, 18:09
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#14
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Marine Service Provider
Join Date: Sep 2012
Location: Virgin Islands
Boat: PDQ 36, 36'5", previously Leopard 45 cat and Hunter 33 mono
Posts: 1,344
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
Quote:
Originally Posted by Searles
The answer is no gelcoat is water permeable under water pressure ie afloat not getting wet on the hard ,anti fouling paint is water permeable as stated that’s how it works ,epoxy on the outher hand is about as close to water proof as you will get on a grp built boat ,the purer the better ,at least that’s how it works in Australia .⛵️⚓️
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And, to the OP, for a 90's era Jeanneau, you are probably looking at an epoxy barrier coat applied over gelcoat, which is a principal reason there are no blisters. Let it be!
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29-05-2022, 18:41
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#15
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Registered User
Join Date: Jan 2015
Location: Gympie
Boat: Volkscruiser
Posts: 2,945
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Re: Once and for all, does the gel coat under the antifoul keep the water out or not?
Green Heaven your over thinking it all. I would say Contrail is right, certainly in our yard in the 1990s we were applying epoxy barrier coats.
Can I say that to me osmosis is always a mystery. I surveyed a yacht the other day and the bow section below the waterline was osmosis free yet the rest of the hull had a section of real bad blisters and then another section of a few blisters. I surmised that when the hull was laid up the guy on the bow section was diligent in removing all the air bubbles in the wet fiberglass while his colleagues did a half job. Reminds me of another boat we repaired and in the laminate was beer bottle tops.
Cheers
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