You can hold a can of dullcoat high, so that the paint falls in a mist creating uneven spots, then sprinkle on brown pastel powders or pigments, it will stick in random areas creating rust pitting. It then also dulls the chrome without being overkill. That is what I have done here.
If you want dull but no rust, you can just skip the pastels on the wet paint, or even use very very fine sanding clothes, like 6000 or even 8000 grit, sand then evenly, then rub on pastel powders in various shades to stimulate dirt and age.
I also had a go at this kit, also weathered, which you can check out here. http://www.modelcarsmag.com/forums/topic/124468-cactus-dan-1978-ford-pickup/
I will be looking forward to more on your project.