Here's 2 video of a test performed by boatwork about the adhesion of polyester gelcoat to epoxy resin.

Enjoy.

Part 1: https://www.youtube.com/watch?v=GwS4YgoGFm8
Part 2: https://www.youtube.com/watch?v=kHAbyglpp3M

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thanks for posting the vids

However i thought it was pretty well known that you CAN use gel-coat ontop of west system and have a good bond. I would have found this much more valuable if he used other epoxies (not just west system) because many of them are know to not bond well.

also when he was ripping them apart he had no way of controlling the forces when he pulled them apart and this could effect results - also he did a single run where he should have repeated it several times to verify results.

I'm pretty sure the information that gelcoat vs. epoxy is a "bad idea" originates from a misinterpretation of what happens in a wet mold and carrying that over into other situations. I haven't tried it, but I can imagine that if you try to do a traditional build using a female mold and spray in wet gelcoat, let that stiffen, and then come in with epoxy that you would probably have a layer between the two where the chemicals fight each other and the gelcoat (or epoxy) never hardens where they are in contact. Actually, I can see this scenario quite easily. I think the community at large has translated this into all sorts of other areas incorrectly.

Here's the thing...once polyester or epoxy resins are fully cured there's no chemical reaction left to it. They're both very stable and relatively inert at this point. Even if you spray gelcoat onto cured polyester resin, you are only getting a mechanical bond. It's only in a wet mold when you lay fabric and polyester (or vinylester) resins ontop of a partially gelled gelcoat do you get a chemical bond.

I've never hesitated to put gelcoat over epoxy ... though I do mostly use West System. With any top coat on epoxy (paint, gelcoat, or otherwise), you do need to be thorough in cleaning the amine blush before sanding or doing any other process. Amine blush is a waxy film that rises to the outer surface of any cured epoxy...it's water soluble and removes easily but if you sand it before washing it, you could just grind it into the surface and negatively affect the subsequent coating process. If I'm doing a large surface, I usually wash it with a light duty scotchbrite pad and some soapy water. Then you give the surface some teeth for a mechanical bond by sanding or whatever.

I would also like to point out that where he put the gelcoat between the two pieces of plywood - one with epoxy and one with poly, that the side that stayed (on the poly), the gelcoat had oozed way over and around the edge of that plywood and probably had a really good mechanical grip around the side. It would have been better if he had trimmed those edges to eliminate that mechanical grip around the edge. There's all sorts of problems with his testing...not the least of which is that poly has significantly lower shear strength than epoxy so the polyester resin, itself, is more likely to fail before anything else. I'm not surprised that his results aren't consistent. Either way, it does show that you shouldn't be scared about putting gelcoat over (properly) cured epoxy.

thanks Jake - good info