My fiberglass days were back in the early 70s when I was building model RC airplanes and it was polyester with wax added so I asked the experts about the Epoxy / Polyester and here is their answer(s). My questions are underlined and their answer follows each question.
System Three Resins, Inc.
1. Epoxy resin will bond to Epoxy resin.
True but the epoxy substrate should be sanded.
2. Polyester resin to polyester resin or to epoxy resin.
Polyester bonds to polyester that has not had a wax added to it to facilitate a surface cure. It does not bond as well as epoxy resin (see #3). Polyester bonds poorly to most epoxy resins. It does bond to our SB-112 (see attached).
3. I was also told that the polyester resin contained paraffin and that is why epoxy would not bond to it.
Only “finish” resin contains wax (see #2). This must be sanded before epoxy will bond to it. Epoxy bonds extremely well to sanded polyester resin – better than polyester. This is why it is the material of choice to repair polyester boat hull laminates.