I've done a few of my cars for a show finish, but you need to know how much paint is on there. With base clear, I shoot 3 coats of base and 4 coats of clear. I block sand the clear to start with 800 grit to get rid of orange peel and any runs. With runs, I put a thin film of polyester putty over the run with a bondo spreader and start sanding with 800 or 1000 grit. That way the run will show through first and the putty will protect the paint around it, until you've got the run virtually out. I block again with 1000 grit at 90 degrees to the 800, then follow up with 1200 at the same angle of the 800, then 1500 at 90 degrees, then you can sand once more with 2,000. I do the 2000 to follow the front to back lines of the body. I have a Trizact sander and sometimes will use 3000 and 5000 on the Trizact (da) before polishing, but you don't usually need to go that far. I use the 3M polishes with my buffer, with diffent pads for each compound. Keep everything spotlessly clean. I do it once with #1, then follow with #2 and #3 and then make a final pass with #3 again in a da polisher to help get rid of any swirl marks. You can skip a few steps, but in the sanding, always follow up with the next grit. You could start with 1200 or 1500 if you're not worried about orange peel. The principal of sanding 90 degrees with each varying grit is so you can tell when you've completely removed the sanding marks from the previous sanding.....that's a critical step. Here's a shot of the door on my '33 five window that was done that way.
