The aircraft from VF-11 above was an experiment in weathering. Using a mixture of browns and jet exhaust from Model Master , I thinned the mixture to a stain consistency, real watery, on a piece of paper a few drops at a time. Remove most of the mixture from the brush, like you're going to dry brush, then hold the brush 90 degrees to your work and pop the brush straight down so the bristles bend and splay making random patterns on the paint. The water consistency is the key. The best brushes for this are the ones that come with Testors paint kits.
Plastic bristles let it really take a beating and still be usable another time.
This mixture is great in an airbrush. Set your nozzle to a superfine spray and you can do panel lines, raised or engraved. It's a great wash for landing gear struts also. You can apply decals and dirty the airframe or dirty it up first, apply decals, then go back and give the decals a wash. I've done it both ways.