When I wanted to create a rusted truck look I went to a model rail road site to get an idea of how to do it. The technique is simply a series of washes of acrylic brunt sianna, burnt umber and some earth tone drawing pastels sprinkled in areas of heavier rust.
On this truck I didn't want the effect to be that sever, so I only used the burnt sienna wash. In the article it says that any base color will work as the original color will be covered up. It suggested just painting the parts flat black as a base. I did this, but then sprayed a little more primer over the parts of the cab I thought would be more bleached out by the sun. My thinking was that in the article the auther was creating a much rustier vehicle than I wanted and I may wish to leave a little of the worn paint on my truck. On this same note I added a bit of turquoise in spots which I think added to the effect on my application of this technique. Okay enough rambling! Here's what I did to the front end...
First it was primered then painted flat black. Then a light wash of burnt sienna. Here the first coat of paint is still wet.I used a hair drier to quickly dry the paint. This is what the first coat looked like once dry. then a few more washes (7 total) drying with the hair drier between each one it looks like this.
This is only the first of 3 steps and I plan to experiment with this some more!