For some reason I couldn't quote your question so you got a notification... Anyway here goes how I do it. There are MANY ways to do this, and you can find a lot of them on youtube.
So, basically what I do is take the body and dent it w/ a lighter... careful not to warp it too much. That gets all your dents and stuff in first before you paint because you'll ruin the paint if you try it afterwards.
once you do that, paint the body with a black base, then a red oxide primer or a gray primer depending on the year of vehicle. Most stuff from the 50's was red oxide. Once that is done, I use a product called "sophisticated finishes" which used to be found in craft stores but not anymore. You can still get it online. It's not cheap, like 30 bucks. But it's great. So you then take the bottle of the thick gray iron base paint and paint on the areas you want rust. Let that dry for a day, then cover it with rubber cement. What I would do then would be to spray some hair spray on it and toss on a little bit of table salt in areas like the roof or hood. Any flat spot that gets weather beaten. Then paint the body with a white or light color, followed by a final coat of your final color. In my case green. then use tape to pull off the rubber cement.
Now you have your ripped/torn paint affect. Then use the 2nd bottle that comes with the sophisticated finishes, and use it to brush over the gray paint. Seal it in a bag for a day and the next day it will be rusty. It takes a time or two to get right. And I have many touchups and whatnot on that body. Then you sand with 600 grit in other spots to go thru like worn paint to the white or even to the red oxide. Then sand where you had salt. Soak the body in water to let the salt dissolve. Shouldnt affect the rust, but if it does you can always add more! Then just play with it from there and you should have a pretty good looking model
Jesse
