Weathering effects can be tricky, but can add a lot to skin, if painted correctly. If you are interested in finding a good weathered look, the first step is research. Find a nice selection of photos, yes, black and white pics will do. Compare all the pictures of your a/c to find the oil stains, gun powder stains etc that are common to each aircraft. Those are the ones you want to render. As always, getting that effect requires layers. I suggest you build up, layer by layer a good smudge with your airbrush tool. Use mulitple kinds of brushes and not just basic 256,256,256 black. Throw some rust colored red in as well, maybe a little grey on the edges where the paint has worn off. Just like panel lines the trick is to not get "out of hand" with these effects
copyed from a WB skining site