Mbounds wrote: "Actually, epoxy sticks quite well to polyester (and almost everything else). It's the other way around - polyester doesn't adhere well to epoxy, especially if the amine blush hasn't been removed."
Not sure I understand this. Whether epoxy sticks to poly or poly doesn't stick to epoxy. I didn't think it would matter which was on top. Just that the two were not compatible. Or is it that poly doesn't stick to anything very well? Thanks
You have to remember that they start out as liquids and end up as solids, so the order does matter. There are also two types of adhesion - mechanical and chemical.
Polyester sticks best chemically to other polyester, otherwise, the surface needs to be roughened up to provide good mechanical attachment. By rough, I mean sanded with 80 grit. The nice thing about polyester is that you can put a new layer on top of another layer that's still "green" - not fully cured - for an even better chemical bond.
Epoxy develops strong chemical bonds with almost everything. But when it cures (especially in the presence of humidity/moisture), it will develop an amine blush
- a thin waxy film that must be removed before any further coating is done. Removal is performed with soap and water, then light sanding to provide mechanical attachment.
If epoxy is properly prepared (amine blush removed, surface roughened), then poly will mechanically stick to it, just not as well as it would sticking to itself (chemical + mechanical).