I've been using my Porter Cable random orbital sander/buffer with sanding discs. They attach with velcro to the sander and are very easy to use.
I started out with 80-grit discs to get down to the primer level then switched to 100 and 120 grit discs to take it on down to bare metal. The higher grit paper really puts a fine finish on sheet metal without scratching it or gouging it like the 80 grit can.
I think I have about 20 or so hours in it thus far. It's a messy job best done outdoors wearing a filter mask of some sort.
I still need to sand the top and skirts, etc.
For the small crevices and places where the orbital can't reach, I've used a hand sanding block. I've been able to get all the metal down to the level of any pitting I've discovered (basically some areas under the side trim) and if it's not too bad, I've been able to go past that level to smooth it out.