I won't do ANY sheetmetal with a sand blaster. Well, if you turn it down as low a pressure as you can and still get some action, you could get away with doing the floor, and some of the higher rigidity areas like door jambs and such. But when in comes to exterior surfaces, especially the broad "flat" ones like doors, quarters, and roof...........................no way Jose'. Unless you want to have your car look like a potato chip ad.

You're smart to avoid commercial blasters who don't have a gang of experience. Personally, I'd do derusting with chemical agents, and I don't mean full immersion. You don't want any of that material in the nooks and crannies, it will bleed out later. Especially bad if it does it after paint. Then follow up the chem action with a DA, preferably soft pad, and don't hold it in any one area very long. Better to go back and forth a bunch of times (less heat build up in the metal), than bear down in one area 'til it shines.