Not sure if your car is assembled, but I shoot all my cars dissasembled, with parts hanging especially on high metallic cars. You get a much more even flow and less chance of any light or dark areas. Once base coat is dry, I wet sand with wax and grease remover before clear coating using 600-800 grit paper. Alot of people will say not to use Wax and Grease remover but for me it works alot better than water because it flashes faster and you know it is clean and not contaminated with the garbage that your water may have in it. Be sure not to sand past your current coat of base. if you sand a metallic beyond the first coat, you will have discoloration and you will have to re-apply another base coat. Then put plenty of clear over it and if you have any issues with the clear, you can wet-sand the crap out of it as long as you dont break through it. If you break through the clear coat then you are in trouble. Here is an example of the process I do on my cars. This car was sold at Barrett Jackson a few years ago for 40 grand Barret Jackson . I painted it in my garage using this process. It was a one of one known to exist numbers matching Auto. 383 Notchback. This was a very heavy mettalic car, but as you see it is straight as an arrow and paint is immaculate. But to answer your question, 600 is good, anything less could end up with scratch marks showing up after clear.