Kind of depends on how much paint/rust you have to remove first to get to bare metal. Are you hand sanding, or using an electric, or an air powered dual-action rotary sander.
They also make those scotchbrite pads for removing paint.
If it had ok paint for example, I have used 200 for a scuff, then primer/sealer, then 400 and/or 600 on that. The sanding primers go on thick and fill up small scratches/sworls.
