Find someone who specializes in restoration blasting, and not industrial blasting. I would caution against using sand, as many types of sand contain caustic salts that, when the metal surface is broken, will start the rust process again, regardless of how clean the panel looks. There are plenty of other blast medias available at a similar cost that will not produce the salt damage issues.
I'd suggest reading up on some of Porchdog's posts on Trifive dot com, he does it for a living and does it well. He also suggests no more pressure at the machine than 35-40 lbs, and hold the nozzle at a 45 degree angle to the panel. Any access holes should be blocked off when blasting insides of door panels, etc, as inadvertant blasting that contacts the back sides of exterior panels can cause stretching/warping. It doesn't take long for someone who doesn't know what they are doing to ruin sheet metal.
