This may be old news, I don't think I've heard it before.
I know it's old advice to use charcoal pencils with weathering, but my main problem was that it always left very clear lines from where it was rubbed, I just couldn't get a nice spread out effect.
Well, for my helmet I actually filed my charcoal onto the helmet using an x-acto knife. Hundreds of little charcoal shavings, spread randomly onto the surface. Just wipe around or in the direction you want the blast to go and wa-la. No clearly defined lines, just a nicely dispersed blackening of varying degrees.
I plan to try it a little on my shoes and jumpsuit, if there's any interest here I'll let ya know how it goes.
Hope this helps,