I would flatsand your primer with 600 grit wet. Then spray on your sealer, followed by three coats of base. At this time, you can let it cure and do your colorsanding. I would probably use at least 1000 grit, maybe even 1200. Depends on what your choice of paint manufacturer recommends. Then spray on a few more coats of base followed by three coats of clear. At this point you can either flatsand the clear with 1500 or 2000 grit and buff it, or you could flatsand it with 1000 or 1200 and re clear the car again with over reduced clearcoat. This is called a flowcoat. You can then flatsand the flowcoat, though you shouldn't need to, or just hit the dirt nibs and buff those spots.
Good, Fast, Cheap, but you can only pick two!