I'm not sure about dealing with the "dimples," but 80 grit sanding, followed by scotchbrite and water should give you a good surface to epoxy over. Chances are that the dimples are caused by some sort of contamination that caused the epoxy to bead up around the area.

Sanding the final coat of epoxy will be somewhat dependant on how smooth the surface is or isn't. But for the final sanding before varnishing I'd use 220 grit. I think a lot of people are finding they can use 100 grit and get a good varnish coat.