Sanding is the prep for paint. Use 36-grit on all bare fiberglass. Then over that, start sanding the filler with 80-grit and work up to 180. The part can now be primed. A good high-build, sandable primer will fill in all 180-grit and up scratches, as well as some of the 80-grit scratches, and will fill but not cover up scratches from the 36-grit. You will want to either prime and seal, or use two coats of primer.
After primer, the part should be sanded lightly with 320-grit. At this point some orange peel should be left and the decision to re-prime or paint must be made. If the decision is to re-prime then sand the rest of the orange peel off with something higher than 180-grit and re-prime. If the decision is to paint, then the piece should be sanded with 400-grit, so that all scratches are no deeper than 400-grit. After this the part can be quickly 600-gritted, and is now ready for paint.
If the piece will be covered then all scratches much be at least 80-grit or higher. 36-grit will sometimes show through.