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
Andy from the youtube channel Boat Works Today has addressed this as well in a couple of videos (and he shoots for Total Boat as well).
That being said.. if i need to make a repair that requires resin, glass and then gelcoat, it's generally easier and much quicker to stick with polyester and gelcoat. Polyester also has the advantage of being easier to spread and works with fiberglass chop strand mat (which is incompatible with epoxy).
If i need to patch something quick and won't be finishing it for a week or more, then I might use an epoxy, like marine tex, which is waterproof but not uv resistant. I can later wash, then sand, then cover with gelcoat.
Although I agree with the video, In practice and for less experienced users when doing layups,
it's simpler to stick with one product (polyester / gelcoat)... so long as only the last coat is waxed! There's less that can go wrong.
For example, I had a docmate make a nice epoxy repair. He got back to it 2 weeks later and proceeded to sand the blush into the area, missing the important wash step.
Ultimately, there are many ways to make a repair, sound technique and sequence is what matters.
If i need to patch something quick and won't be finishing it for a week or more, then I might use an epoxy, like marine tex, which is waterproof but not uv resistant. I can later wash, then sand, then cover with gelcoat.
Although I agree with the video, In practice and for less experienced users when doing layups,
it's simpler to stick with one product (polyester / gelcoat)... so long as only the last coat is waxed! There's less that can go wrong.For example, I had a docmate make a nice epoxy repair. He got back to it 2 weeks later and proceeded to sand the blush into the area, missing the important wash step.
Ultimately, there are many ways to make a repair, sound technique and sequence is what matters.
How about using vinylester resin? It makes a much better secondary bond to polyester, is far more impervious to blistering (below the waterline), and exhibits the same characteristics as working with polyester resin.That being said.. if i need to make a repair that requires resin, glass and then gelcoat, it's generally easier and much quicker to stick with polyester and gelcoat. Polyester also has the advantage of being easier to spread and works with fiberglass chop strand mat (which is incompatible with epoxy).
If i need to patch something quick and won't be finishing it for a week or more, then I might use an epoxy, like marine tex, which is waterproof but not uv resistant. I can later wash, then sand, then cover with gelcoat.
Although I agree with the video, In practice and for less experienced users when doing layups,
For example, I had a docmate make a nice epoxy repair. He got back to it 2 weeks later and proceeded to sand the blush into the area, missing the important wash step.
Ultimately, there are many ways to make a repair, sound technique and sequence is what matters.
I had my local boat yard use it for a couple of blister repairs during my haul out at the end of last year. No problem at all obtaining vinylester resin out here (Southern California), and while it does cost more than polyester, it is cheaper than epoxy (or WestSystem, at least).
I intentionally stayed away from that point to stay on topic of gelcoat over epoxy, but now that you brought it up...
The use of epoxy for repairs is one of those "everyone knows it, because everyone keeps repeating it" things. Epoxy is almost never necessary for repairs. Maybe for something like retabbing a bulkhead, but even then, not really. It should be used for carbon fiber and when light layups are needed, but this describes very few boat repairs.
Epoxy is extremely toxic, if you become sensitized to it, you will be so for life, it is difficult to work with properly, does not allow much option in curing or pot life, slows projects down by days waiting for cure, does not wet out laminate as easily, requires a lot of post laminating cleanup/work, and cost 5x more. In short, everything about working with epoxy works against one in doing a boat repair.
FWIW, I've done more work in epoxy than polyester, so this isn't a view from lack of experience.
And I know all of the arguments about secondary bonding, etc, but I and others have done a lot of repairs, both large and small, structural and not, in polyester that have never failed. And never will fail. The arguments for epoxy are true in the theoretical sense, and true for a small percentage of application, but untrue in the practical sense for most applications.
Mark
Vinylester is great. However, it is a bit persnickety to use because it requires a narrower range of environmental conditions, closer adherence to mix ratios, and the shelf life is very short. The latter isn't an issue if you buy it fresh and use it right away, but always check the date - I've seen it gelling on the store shelf.
Mark