For metal body work? The truck has missing paint and mild oxidation all over the cab. I was gonna sand it down and spray some primer on it for now. What kinda sandpaper should I be using?

I'm not concerned with getting a real high quality finish, I just want to get rid of the oxidation and the primer to stick, so preferably just one grit for the whole job rather than 400 and 800 or whatever... I'm a nub to paint prep.