Yep right over the epoxy. If after 72 hours you may need to 150 the epoxy for the filler to stick. I have found that 80 grit paper works best on the Xtream for first cut.
If you use a courser grit it tends to rip the filler from the panel if you get on it before it is completely hard.
Another tip it is try 3M Hook-it paper, it is Velcro backed paper. It cuts well and can be removed and reused.
Dave