West System Epoxy incompatible with Gel Coat?

[movesnomore]

I think I screwed up.

I'm helping my son repair his Hobie 16 (long story) and we were advised to use West System epoxy. We're almost halfway done with some extensive hole repairs and re-fairing. Sanded thru half the old gelcoat. Almost ready to paint. Pretty good job for amateurs

Problem is we had planned on using gelcoat. I was sure it would work... I even read the West System book (not well enough, I guess). I was reading in this forum about Roll and Tip method when I saw this chilling comment "Gelcoat over epoxy, NO.You can epoxy over polyester, but not polyester over epoxy."

I called West Marine tech support and they recommend no gelcoat but to use paint like Interlux Perfection or their brand.

Not sure what I'm really asking the forum... maybe to confirm above, maybe to vent a little...

[TeddyDiver]

Confirmed.. feel vented

[gonzo]

Gelcoat will peel off from epoxy

[movesnomore]

Wait - now I'm reading from West System's web site that gel coat applied to their epoxy works just fine... http://www.westsystem.com/ss/assets/...2Polyester.pdf

Very confused

[ondarvr]

I've actually called them to discuss this claim, they said, well, it may not be the best method.

Here's the deal, we make and formulate gel coat and say don't do it, they sell epoxy and say it "may" work.


I've tried it myself and was never able to get a bond that I thought was acceptable.

[PAR]

Yep, the major formulators are beginning to produce a primer that permits gel coat to stick to epoxy repairs. I've only seen it once, never have used it (just don't trust it yet) and have read about it. Chemically it's questionable if the bond will be particularly durable.

Jimbo, have you any experience with type of product?

[ondarvr]

We make a tie coat and have for many years, but I can't say its a big seller and I haven't used or sold any to know just how well it works.

[AroMarine]

The biggest problem with West and gelcoat is the cure of the epoxy. The ammines are that are emitted even after it feels hard and and the physical changes of ongoing curing ruins gel cure and bonding. That said I have gelcoated over West for years in small enough areas that I was able to heat adequately. When you have gotten to the point of where you would apply gelcoat I scrub my repair with water and heat using a heat gun. You want to heat your repair to 130-150F Just to the point where you can maintain touching it. The longer you can heat your work the better chance you have of getting your epoxy to fully cure. Water is the solvent for ammine do not use acetone etc. After two or more water scrubs( I use green scotch brights) and a minimum 150 grit but preferrably 80 grit sanding you shold be good to go.

[ondarvr]

I've tried it over new and old epoxy surfaces that were sanded with 80 grit and thoroughly cleaned. What I found was it will stick, but not nearly as well as it will to any type of polyester surface. If you need to chip a bug off the surface or clean up an edge with a putty knife, a chunk can pop right off the epoxy. What you see on the back side of the gel coat is a perfect mold finish of the sanded epoxy surface.

I'm sure that different epoxies will have different results and so will different types of polyesters, so I can't say that there may not be a few combinations that may work much better than others, I just won't recommend it and neither do the chemists in our lab.

[Knut Sand]

Originally Posted by ondarvr

.....but not nearly as well as it will to any type of polyester surface. If you need to chip a bug off the surface or clean up an edge with a putty knife, a chunk can pop right off the epoxy. What you see on the back side of the gel coat is a perfect mold finish of the sanded epoxy surface.

Been there, done that....

[rocknrule]

I've heard that you can build a epoxy/glass boat on gel coat (in a mold) if you start using a very thin poly/glass laminate on top of the gel coat and allow it to cure. Once cured you can do your full epoxy/glass laminate. I am not a fiberglass person so this is only hearsay - Is it true? Is the laminate good/durable?

[blonhrt88]

So what did you end up doing instead? Good thing I ran into this thread...

[Jimbo1490]

Originally Posted by PAR

Yep, the major formulators are beginning to produce a primer that permits gel coat to stick to epoxy repairs. I've only seen it once, never have used it (just don't trust it yet) and have read about it. Chemically it's questionable if the bond will be particularly durable.

Jimbo, have you any experience with type of product?

This type of product has actually been around quite a while now, but it existence is just now becoming known to 'mainstream' users. Adtech made the first one I'm aware of, and I had some experience with that product back in the early 90's when I was making go-fast car hoods.

But do you really need a special 'interface' product for this?

Cured epoxy is pretty inert stuff, very resistant to chemical attack, and therefore any adhesion that might arise therefrom. So you have to sand a cured epoxy surface to promote mechanical adhesion of any subsequent coatings. At that point, what's to stop you from using any of several polyester based primers, then re-coating this with gelcoat? The primers will adhere to sanded (or better yet, peel-ply) cured epoxy, the epoxy is not going to dissolve or even soften, gelcoat will be compatible with polyester primer and therefore all should be right with the world.

The tricky problem that were were trying to solve is to spray gelcoat into a mold, then do an epoxy layup on top of that. Now that's tricky, but that is what the Adtech product facilitated. Poly resin is really not compatible with carbon fiber (don't let anyone tell you it is) and we wanted to sell a hood that had a 'finished' gelcoat look, without laborious post-finishing.

Jimbo

Systems Three SB112 was advertised as being poly-compatible. We matched existing gelcoat in an engine room after replacement of shaft logs using this product in, I don't know, 1996?. I was in that engine room this spring and the gelcoat looked fine. Nor has the gelcoat flaked off of the bottom.

[movesnomore]

What did I do instead? Well I decided to try the gel coat on the rudders first instead of the hull. Glad as hell that i did. GFelcoat seems to adhere pretty well so far, but we learned quickly that we're not using gelcoat on the hulls.

We may have done it wrong...who knows. Everything i read says apply 2-3 coats to a well prepped surface to about 20mils thick. 2-3 coats barely got me any thickness. I applies 5 coats with wax in the last. I used a full $16 quart on the 2 rudders. when going thru the grits, i sanded thru in some small areas and almost thru in several others. And I was careful. Bottom line is we would have had to spend about $600 to complete both hobie hulls. It's just a family boat, so we're using Brightside instead over their Primecoat primer for less than half that and far far far far far far far far (did I make that point well enuff?) less sanding.