Ok, started work on my 40 Ford that will be modeled as if just fresh pulled from the field and going to the shop for a restoration.
I got this idea and proceedure from the link Syxx Killer provided.
First I distressed the plastic body with a dremel tool. I created rust holes were needed.
next I painted one light coat of flat black and added salt on top. I then sprayed a coat of brown primer with will serve as some rust color but not all. I added another bit of salt to the paint.
Once this dries I'll apply a product that creates actual rust. I will also apply my final color which is blue. There will be an application of rubber cement over the rusting material. You will understand why later on. I will post up pictures when I do this. After the last coat and the rusting product I need to allow dry for the night.