Can you be a little more specific? There is a lot of "weathering" that can be done with pastels. I use pastels as a final step. I take what ever color would apply (Brown for dirt, Grey for grimme, tan for sand etc.) and simply brush it on with one of my wifes make-up brushes. To make chalk dust, I simply run the color chalk I want over some sand paper, collect the dust in a jar. If I want a little "thickness" I add a drop or two of water. I try to avoid oil based pastels, as they tend to leave streaks.
As for rust streaks, you can combine "rust" colored pastel chalk with some brown paint to make a paste consistancy. Apply this paste with a thin brush where you need it. You can then make a wash with either red brown or rust color and allow the wash to run in the direction water would flow off the rust spot.
Does this help?