Wow!
Talk about a loaded question! There are a thousand if, ands or buts to a helpful response. I'll toss in a few comments and others, I am sure, will add more. They may not be as contradictory as some will seem. You don't say whether you are painting over paint, wood, steel, aluminum, cement or fiberglass. The harder the material, generally the coarser the grit for prep. 220 grit is usually fine for oxidized fiberglass. 400 grit is for prepping when you are going after a flawless finish. Fill the gouges and dings and sand flush with a long flexible block. Every lump and defect will stand out under a glossy finish like a flashing neon sign. Sanding over new paint is not for shine, it is for removing defects. It shouldn't be done until the paint is fully cured. It is often started with wet sanding using 600 grit paper and progressively moving to finer grits such as 1000 (or better if you can get it). Keep the water bowl clean and use absolutely fresh water and an immaculately clean bowl when moving up in grit. Then you will typically move up through progressively finer grades of polishing compound until you are satisfied. Satisfaction is in the eye of the beholder. A careful roll and tip job looks pretty decent from 10' away( this is the meaning of the 10', 6', 3' rule) with no follow-up sanding. A pro spray job followed up as described above, looks great from 10" away. Leave Awlgrip spraying to the pros, the overspray is highly toxic and requires specialized equipment (roll and tip is OK).
