You need to sand it with a 60 grit paper and then kiss the surface with an open flame from bunsen burner or a torch. You can check how the surface tension changes with a drop of water that forms nearly a sphere before the flaming and a big spread out blob afterwards.
Epoxy will not attack ABS. Polyester will because of the styrene in it.

I'm not a chemist and not really qualified to elaborate more on what's going on exactly, but i understand that the surface is oxidizing. I have my doubts that the heat gun is enough heat as all the industrial settings i have seen used a torch to modify the surface. but this is typically something you want to make a quick test yourself anyway to test the process. so grab some ABS, a heat gun and a lighter and see how it goes.