feather out all cracked paint with 320/400. if you can feel it with your finger, you'll see it in the paint. if you can feel the feathered edge, you'll see a ring in the paint. then scuff well with a red scotchbrite pad until part is 100% dull, paying more attention to edges/high impact areas. You should probably ask the guy who is painting them what he prefers. with 320, he'll have to seal them, if you keep it under 600, sealer isn't needed.
Hope this helps
BTW, what are ya paying for paint, if you dont mind my asking?
94 V8 on 17" custom paint, R.I.P
97 Mark VIII LSC yeah
08 Ford Edge, white for now