"Rust-reformer" by Rust-Oleum in the jug works well. It uses a zinc formula to neutralize and seal the metal so it can no longer oxidize. CRC "rust converter" is the product that claims to leave a primer surface it works similar to the Rust-Oleum product in effect. I have found that both of them work the best with an etching type primer before any acrilic finish coats.
Now, this is of course usually withheld at my place for the trucks headed out to the dunes or the mud runs. The finish you will get over either product looks textured, just like the depth and grain of the rust it covered.
When I lived in Michigan I had a monster bronco basically held together by cookie sheets glued on with 5200 marine adhesive and set with rivets. Most of the remaining body was hit with a flapper wheel, stripper disc, primed with a zinc based primer and forgotten about. The second half of that is how I would go about it if you're good with the "10 foot rule". Just the rust reformer usually produced a finish that is "parking lot good" or as I have also heard it called "good enough for a bar pick up" because clearly only a drunk would get in that car with you.
