Sanding steel to spraypaint it?

what grit of sandpaper should i use to sand my bike? it's chromoly, and I want to spraypaint the seatpost/pedals/frame/fork, a cobalt blue. the pedals are alum-alloy. I'm going to leave the mainframe chrome. but yeah, I have 320 extra fine grit laying around... would that work? or what do you suggest?