If the fiberglass has already been painted and you just scuffed it and did not break the surface, the sealing operation can be skipped. The only reason I don't recommend skipping the sealing operation is that I would rather have something go wrong (fish eyes/ contamination) in the sealer coat than in the color coat. #600 if you skip the sealer. #400 if you are sealing it. Single stage is more forgiving than base coat/ clear coat on the sand grit. You could get away with #400 without sealer with single stage.
