I just finished weathering these 2 cars for a client.
Along with the usual air brush oversprays, I used a relatively new product for a few of the streaking effects.
Pan Pastels weathering colors. Similar to weathering chalks, but far less messy to use. And easier to control.
I had seen a clinic on this material a couple of years ago in Naperville and at the time discussions were underway with the manufacturer to release a set of "weathering" colors. I picked this set up a little while ago and fiddled with them some. I'm very pleased with the results.
This medium will now become a permanent part of my repertoire of weathering materials.