This is what I would do...
1. 400 wet sanding and sand it just enough to get gloss off.
2. Grease remover
3. Spray light layer of primer/primer filler.
4. Sand to smooth, because primer leaves unsmooth surface. You might have to use dry sanding first(~400) and then wet sand it by starting 400 to 800 if you like. This sanding sets the final surface, so go smooth. Dont wear out the layer.
Skilled sprayer gets the primer surface quite smooth.
5. Grease remover
6. Actual painting
7. And so on...
Im speaking only from my own experience. Im no professional painter.
Are you doing actual painting your self? Are you using paint and clear coat? Or (HS) paint and clear coat combination? are you using 2component paint? What are you painting sideskirts? just out of interest.
I have been happy with Spies & Hecker products, I dont know can you guys get hand on these? Spies & Hecker provides almost everything. Use only good quality sanding paper like 3M, use low quality paper and you see why I said this .
I assume that primer filler means thick primer not spray putty.