Steel's great - you just can't kill it! OK, so the thin stuff might rot through, but the meatier components can be wire brushed, emery-clothed, dunked in acid, even kicked around with steel toe-caps, and still they're ready for another lick of paint. Quick dose of primer, chuck on a top coat of colour and everything's cool.
But Aluminium (or Aluminum if you're American), is a different story.