I’m starting a fin change and ding repairs on a old board Its laminated with polyester resin, can I use epoxy with the polyester or keep it the same
Yes, you can.
It’s the opposite (polyester on a polystyrene/epoxy board) that can’t be done.
But why not use good old (and cheap) polyester?
I’ll add that I prefer to live by the rule “fix it with the same materials”, whenever possible.
And when I have to deviate from that I usually go begging to doc for info.