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Old 14-02-2016, 13:46   #16
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Re: Gelcoat can be successfully applied over Epoxy repairs

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That's epoxy on poly, should be golden.
Music to my ears.......
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Old 14-02-2016, 16:49   #17
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Re: Gelcoat can be successfully applied over Epoxy repairs

polyester to polyester has a chemical bond, not just mechanical. epoxy bonds mechanically. they also expand and contract differently, making a mechanical bond of the two different plastics break. Repair poly with poly it's stronger. filling holes? use a thickening agent as mentioned above. Epoxy can go on as a barrier coat over poly without a problem it is more flexible and won't loose its bond, no need to gelcoat as its underwater.

the only time I've seen gelcoat over epoxied glass was when it was primered with paint and that was on a flight simulator, indoors. maintained good at a constant 75 degree air temp.
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Old 15-02-2016, 03:16   #18
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Re: Gelcoat can be successfully applied over Epoxy repairs

I'm curious why my post about the importance of washing epoxy and a thanks to the pros who share info here disappeared? was removed?

I'm no pro, but I have several years of selling boat products and I've discovered that do-it-yourselfers often are unaware of amine blush and the requirement for proper washing. More experienced users probably take that knowledge for granted.

And thanks to Minaret who answered my query about 545.




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Old 15-02-2016, 05:36   #19
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Re: Gelcoat can be successfully applied over Epoxy repairs

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Originally Posted by tinkrman69 View Post
polyester to polyester has a chemical bond, not just mechanical. epoxy bonds mechanically. they also expand and contract differently, making a mechanical bond of the two different plastics break. Repair poly with poly it's stronger. filling holes? use a thickening agent as mentioned above. Epoxy can go on as a barrier coat over poly without a problem it is more flexible and won't loose its bond, no need to gelcoat as its underwater.

the only time I've seen gelcoat over epoxied glass was when it was primered with paint and that was on a flight simulator, indoors. maintained good at a constant 75 degree air temp.
Actually stronger repairing polyester with epoxy.
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Old 15-02-2016, 09:01   #20
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Re: Gelcoat can be successfully applied over Epoxy repairs

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Actually stronger repairing polyester with epoxy.


Why would you want it to be stronger than the original material? Dissimilar materials in a laminate are not your friend. Don't make hard spots with your repairs. Not much point making a joint with a stronger bond if that joint is going to see dramatically more stress because the repair flexes at a dissimilar rate to the surrounding substrate.
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Old 15-02-2016, 09:07   #21
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Re: Gelcoat can be successfully applied over Epoxy repairs

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Why would you want it to be stronger than the original material? Dissimilar materials in a laminate are not your friend. Don't make hard spots with your repairs. Not much point making a joint with a stronger bond if that joint is going to see dramatically more stress because the repair flexes at a dissimilar rate to the surrounding substrate.

Let me rephrase that. Using polyester as a secondary bond on top of old polyester would be substantially weaker than the original.


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Old 15-02-2016, 09:10   #22
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Re: Gelcoat can be successfully applied over Epoxy repairs

Just to add to the chorus of problems with gelcoat over epoxy...

Unhappy with the cosmetics, I wanted to re-do with gelcoat, a few spots where I had repaired small areas with thickened epoxy, then painted over. I removed the paint, and sanded down to epoxy.

This epoxy had been curing for over two years.

On advice from this forum, I scrubbed the epoxy with acetone and then hot water, to remove any amine blush.

The gelcoat that I placed over the epoxy had not cured after a week. The gelcoat that overlapped old gelcoat had cured in 2 hours. I had used gelcoat with wax pre-mixed, so tried again with unwaxed gelcoat, sprayed over with PVA. Still no cure.

So while I am willing to believe that gelcoat over epoxy works sometimes, gelcoat over polyester works all the time, and why have the uncertainty?
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Old 15-02-2016, 09:13   #23
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Re: Gelcoat can be successfully applied over Epoxy repairs

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Let me rephrase that. Using polyester as a secondary bond on top of old polyester would be substantially weaker than the original.


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You think the original had no secondary bonds? It was built in 24 hours? Not the case. Poly iso bonds on ortho production resins are the sweet spot for this. Epoxy creates a harder, stiffer layup than the surrounding material, which creates problems. Work in one resin system. Then you have a single product instead of two. And you can match the original finish instead of having to refinish the whole boat. Haven't seen a single failure yet on any of the thousands of structural laminates I've done this way.
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Old 15-02-2016, 09:28   #24
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Re: Gelcoat can be successfully applied over Epoxy repairs

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You think the original had no secondary bonds? It was built in 24 hours? Not the case. Poly iso bonds on ortho production resins are the sweet spot for this. Epoxy creates a harder, stiffer layup than the surrounding material, which creates problems. Work in one resin system. Then you have a single product instead of two. And you can match the original finish instead of having to refinish the whole boat. Haven't seen a single failure yet on any of the thousands of structural laminates I've done this way.

Yes original polyester fiberglass boats have secondary bonds, and a good history of failed secondary bonds.


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Old 15-02-2016, 10:14   #25
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Re: Gelcoat can be successfully applied over Epoxy repairs

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Yes original polyester fiberglass boats have secondary bonds, and a good history of failed secondary bonds.


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As I said, haven't seen one yet. And I often do poly on poly repairs in major structural work. As in, the boat went on the rocks and is missing large portions of the hull type work. Don't know a single pro who would approach such a major structural repair in epoxy. Not one. And I went to school for this, have a degree in it, and know just about everybody worth anything in the business in my area. Every failed secondary bond I've ever seen was due to a total lack of prep work, because they were actually trying to get a primary bond and missed the window. Never seen a single failed secondary bond with grinder marks under it. Ever. This means that wasn't a secondary bond at all.
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Old 15-02-2016, 10:37   #26
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Re: Gelcoat can be successfully applied over Epoxy repairs

Same here, I love work with Epoxy, but we work mainly with Poly, to match OEM materials and to avoid dismilar materials in tricky spots, hard spots... Last large job, a 5 ft stern extensions in a multi, made in poly and gelcoat, secondary bond obviously .. no problems at all.....
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Old 15-02-2016, 10:38   #27
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Re: Gelcoat can be successfully applied over Epoxy repairs

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As I said, haven't seen one yet. And I often do poly on poly repairs in major structural work. As in, the boat went on the rocks and is missing large portions of the hull type work. Don't know a single pro who would approach such a major structural repair in epoxy. Not one. And I went to school for this, have a degree in it, and know just about everybody worth anything in the business in my area. Every failed secondary bond I've ever seen was due to a total lack of prep work, because they were actually trying to get a primary bond and missed the window. Never seen a single failed secondary bond with grinder marks under it. Ever. This means that wasn't a secondary bond at all.

I boat builder I worked for would lay the hull up in polyester, everything after that was done in epoxy. The charter boats he built have an outstanding record for no structural problems, and they are used and abused daily. Like I said, seen to many failures of polyester secondary bonds so it's something I would never do.


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Old 15-02-2016, 14:06   #28
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Re: Gelcoat can be successfully applied over Epoxy repairs

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Same here, I love work with Epoxy, but we work mainly with Poly, to match OEM materials and to avoid dismilar materials in tricky spots, hard spots... Last large job, a 5 ft stern extensions in a multi, made in poly and gelcoat, secondary bond obviously .. no problems at all.....



Great point. I too have done many extensions, power and sail. At several large and renowned companies. Never seen one done in epoxy, never even seen it suggested by a pro.
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Old 15-02-2016, 16:05   #29
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Re: Gelcoat can be successfully applied over Epoxy repairs

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Great point. I too have done many extensions, power and sail. At several large and renowned companies. Never seen one done in epoxy, never even seen it suggested by a pro.

When we extended our Seawind 1000 the pro boat builder insisted on using epoxy, worked out well as I insisted on epoxy as well.


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Old 15-02-2016, 17:13   #30
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Re: Gelcoat can be successfully applied over Epoxy repairs

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When we extended our Seawind 1000 the pro boat builder insisted on using epoxy, worked out well as I insisted on epoxy as well.


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I'm curious what you then used as a topcoat for your extensions. Gel? Paint?

In general, if you use epoxy for minor repairs on a gel deck, can you bypass using 545 if you then use an epoxy based paint to try & match the gel? For minor stuff, I've used epoxy, but also gel with some Cabosil mixed in as a filler/thickener. So long as I got the color matching right (terrible at first but improving . . . maybe), then using gel alone seemed to eliminate a lot of the work & uncertainty of getting gel applied over epoxy.
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