From time to time, SBO members as if gelcoat can be applied over epoxy. This question usually comes up in a discussion about hull repair and whether the repair must be done with epoxy or polyester resin. The answer to the question is, yes gelcoat can be applied over properly prepared epoxy. West System recently released a video on this topic and demonstrated there is little to no difference between gelcoat adhesion to epoxy or polyester.
Gelcoat over Epoxy
- Thread starter dlochner
- Start date
Good to see this myth put to rest. I've used gelcoat over epoxy many times with no issue, but I've stopped trying to convince others because of the strongly held myth that it can't be done.
Just make sure the epoxy is fully cured, any blush is removed, and tooth it aggressively. I didn't watch the video, so they may suggest differently than this, but I've had no problem with my method.
I'd nitpick that there is no difference in adhesion. I'm fine with the conclusion that adhesion won't be an issue, but if gelcoat is applied over laminating polyester (or vice versa if using a mold), there is a strong chemical bond between the two that does not exist with epoxy. With epoxy, it is all mechanical bonding.
But this mechanical bond still works in practice.
Mark
Just make sure the epoxy is fully cured, any blush is removed, and tooth it aggressively. I didn't watch the video, so they may suggest differently than this, but I've had no problem with my method.
I'd nitpick that there is no difference in adhesion. I'm fine with the conclusion that adhesion won't be an issue, but if gelcoat is applied over laminating polyester (or vice versa if using a mold), there is a strong chemical bond between the two that does not exist with epoxy. With epoxy, it is all mechanical bonding.
But this mechanical bond still works in practice.
Mark
I am not a chemist, but I believe that chemical bonding of polyester to polyester happens while polymerization is still occurring - while the resin is still 'green'. This happens when gelcoat is sprayed in the mold and then polyeaster resin (and fibreglass) is applied on top. On repairs of cured fibreglass, you are dealing with straight mechanical bonding, which is why epoxy is preferred for its superior adhesion. So, applying gelcoat to either type of substrate is pretty much the same.
I don't disagree, however, the video seems geared towards repair work done by DIYers and yards who prefer to use epoxy for repairs.
Using composite resins throughout my career, I was always taught that chemical bonding occurs when multiple layers of resin are applied to an uncured substrate; however, once the resin is completely cured & amine blush is removed, additional layers form a mechanical bond only. That’s not to say that mechanical bonds are insufficient for a specific task; however, bond strength will never approach that of a true chemical bond.
My point wasn't about the secondary bond to existing fiberglass, it was about the bond between gelcoat and new fiberglass (the repair itself). If using polyester for a repair that will be finished in gelcoat, one uses a laminating resin that does not have any wax in it. Polyester cure is air-inhibited, where without a surface coating blocking the air, the exposed surface never cures. So applying gelcoat a day or two later still allows a chemical bond because the surface of the fiberglass still has unreacted molecules to polymerize with.
Boat builders use this to great advantage, where they can spray a mold with gelcoat, then let it fully cure for a couple of days, then start laminating over it. Lamination can be stopped at any time, and picked up later, and still have chemical bonding. Multiple laminates can be done over time without worrying about preparing the surface each time.
But in practice, repairs are usually done in dirty conditions and require a lot of grinding, etc, so relying on air inhibition for a chemical bond is mostly ignored. However, I've often coated a repair area with a thin coat of laminating resin, let it cure, and applied gelcoat for the chemical bond.
Mark
Yes, I wasn't advocating polyester for the repair. I was just nit-picking that gelcoat will always be a mechanical bond to epoxy, where it can be a chemical bond to polyester.
Mark
For a finish, a mechanical bond is enough. After all, nobody blinks twice at painting an epoxy repair, but for some reason gelcoat over epoxy became a big no-no myth. gelcoat really isn't much different here than a 2-part paint.
Personally, I think the myth started from influential people who had little experience with composites, and particularly epoxies. Then it just spread as gospel from there, and other influential people repeated it without actually trying. The problem only occurs with a blushed, insufficiently cured, no tooth surface.
Mark