I was talking to the sales rep from a company called Evapo Rust last week. They sell this really cool rust remover that is safe on your skin, biodegradable, and is realatively cheap. He told me that when the Military tanks come back from Iraq, they submerge big parts of the tank in a giant vat of their stuff they sell to them. I asked where they do that but he didn't know. so I asked him if anyone does this and he told me there is someone in Michigan that does it. He couldn't recall his name at the time as he was on the road from a trip.
He said that this guy has an irrigation system that sprays it throughout the vehicle in all areas for 18 hours. It is in a recirculating system that catches what drains off, is filtered and continually keeps spraying onto the body. The results are that all rust is removed even in the nooks and crannys without removing any sealing material. Whats the good reason for using it is that it doesn't have any negative reaction with primers and paints unlike other types of body paint cleaners and strippers. It doesn't remove the factory sealers either. Your metal is kept natural, no possible warpage from blasting. When you do paint your car, sand won't return and find its way onto your new paint. Make sure paint is removed first.
Considering all the possible options of preparing a car to be primered, this sounds like so far the most logical choice I have heard yet. He said also that after the rust removal process is over, you spray the car with a metal cleaner mixed with water and dry it down with clean dry air. Then he said they have this stuff that is a rust inhibitor spray for metal. Spray that on after cleaning and it'll give you enough time to primer before the rust flash begins. He said the inhibitor will give you plenty of time to primer.
The conversation ended that he told me that Auto Zone currently carries it and Advance Auto will shortly.
I think it should be relatively easy to make a system in the backyard with plastic to do this. 20 or 30 gallons should do it.