Bondo on epoxy?

Hanaldo (05/04/2016)

f1rob (31/03/2016)

A polyester will stick to an epoxy a lot better than a epoxy will stick to a polyester even thou the epoxy is stronger

Never had any problems but surface prep is important

Not to hijack the thread too much, but you've brought this up at an interesting time for me Rob. I have been discussing the issue of epoxy/polyester bonds with some friends in the gliding scene, one of whom used to make fibreglass boats for a living. For reasons I can not fathom, he absolutely swears by polyester resin. He considers it the be all and end all of composite resins, and refuses to work with epoxy.

One of the reasons he gave for polyester being better is that it will bond to epoxy, but epoxy won't bond to polyester. Something that makes absolutely no sense to me, considering polyester is extremely poor at bonding to anything including itself; whilst epoxy is one of the most widely used adhesives around - because it's bloody good at sticking to things.

Now I tried to do a quick bit of research on this topic, but literally EVERYTHING I come across backs up my belief that epoxy bonds to polyester better than the other way around. So I'd love to hear your thoughts on that?

Keying the surface and wiping with solvent will allow either resin to bond or glue the parts together. Think of gluing two sponges together with either resin. The resin will migrate into every pore and you will have a mechanical bond that is only as strong as the cured resin. Epoxy I believe has more tensile strength than polyester so would be stronger. Your boat builder probably likes polyester better because for days after the cure you can apply more resin cloth layers and it will co cure and crosslink with the surface because the surface layer is not cured. (air prevents this unless you add a chemical) This next layer doesn't bond it crosslinks making it as one. Now epoxy will not do this if you leave it overnight. You can only make a mechanical bond to its surface. But if you allow an epoxy part to cure to only a tack free state you can add another layer and it will crosslink and make it as one. This window of time with epoxy is very short and harder to hit than polyester.
Two Cents.
Fred