What grit of sandpaper should be use to get down to the primer on a car?

220 3m its a finer sand paper you can take the paint as low as you want with it it works great. You should always prime over a 220 grit scratch. It is the perfect size grit. If you are sanding primer so you can apply topcoat, finish with 400grit dry on a machine (DA) or 500 wet sand for a solid color enamel. Basecoat products, particularly metallics and pearls, need to be applied over a substrate that has been wet sanded with 600 grit.