Hi Erez;

1st off, polyester resin sticks just fine over epoxy, it's done every day. I have PR on my hull and I can guarantee it sticks tenaciously. The mechanical bond is at least as good as varnish, probably much better.

Next, Polyester resin will chemically bond to itself until a wax/styrene mix is incorporated into a resin coat, at which time you will revert to a mechanical bond.

Lastly, make sure you mix the wax/styrene into the resin THOROUGHLY before catalyzing, otherwise you'll learn to really hate sanding . . .

Best Regards, Spidey