sanding or wheeling doesn't get as clean, grinding makes for a good amount of "texture", grind it to shiney metal - if that leaves it very thin it needs to be cut back further
if you can keep the cleaned/derusted areas dry you don't need to coat them immediately, do as much as you can each session, protect it til next time, get a largeish area done [like one side] and epoxy prime it - rattle can primer won't protect metal, it's not water proof - anything you put on will have to be resanded before the final job, so use the best you can afford
Bill
