Mark, If it worked for you before, I would stay with it.

I don't use epoxy, so I can't offer any proven advice. What I can say is that I always sand to bare metal for polyester filler. I never use it on top of paint of any kind. The only product I use over paint is Fiberglass-Evercoat Euroglaze, and I rough sand the surface before I use that.

You need to have confidence in your paint system, so I would go with what you know!