Years ago I attempted to make an undercarriage look like a car that had sat in a damp junk yard for years before being dragged out and turned into a stock car. IMHO, looking back I think I overdid it somewhat, but the material I used was real rust - I took some steel wool, and placed it into a jar with some water in it (you could make a salt solution to speed up the process). I let it sit in the open jar for days or weeks, letting the steel wool rust. Eventually the water dried up, leaving behind rust powder that I could further manipulate into a fine powder. I also found some rusty steel car parts that I had left out behind my back step (just by happenstance) and sanded off some of the rust from those parts, collecting the powder into a baggie. I painted the undercarriage flat black, and then sprinkled the rust powder onto the wet paint, brushing off the excess after the paint dried. The pic below illustrates that I was a little heavy handed IMHO, and if I do another one I will apply the rust more judiciously the next time around, so the rust is accentuated, not just one big rusty blob, as this appears.

I don't consider the photo below as a tutorial on how to do it, but just an example that the tone/finish of real rust is perhaps the most realistic way of achieving the look. However, practice makes perfect, and to be honest, I never perfected the technique.