for the surface rust i would try some either dupont or ppg rust remover, not quite sure of the name but its a chemical you wipe on surface rust like you have and it comes right off then you neutralize it with water when your done, then whip off and prime.
Using a DA or just sandpaper for that matter will not get the rust out of the lil pits, you wont be able to see it, but rust is still there. Just a though, oh and for bare metal make sure to use epoxy primer or etch, they are both designed to stick well to bare metal, i would use epoxy if i was you. They showed us it at school and i was amazed I though meh use a grinder, the wipe method is much easier and doesnt leave scratches. And i wouldnt probably soda/media/sand blast it, roofs are week and thin, it would probably warp it, unlesss MAYBE you used a 'spot' blaster and wanted to do it all day.
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