Here's a suggestion I haven't seen anywhere yet. In addition to using chalks for weathering purposes try a similar method with (artists) charcoal pencils. They can be scraped into a cup and applied using a brush, or use it like a pencil for fine work. The effect is more of a 'grimy', dull black. Its also a little more premanent than chalk, although it can be wiped off with some work. Try it. Looks great!