In the past I’ve used both an airbrush and chalks. With the airbrush I pick three colors, white grime, rust and grimy black. White grime goes first and you fog the building with it. Next rust color is added for anything metal like ladders or sheet metal and then you streak with soot using the black. I don’t bother to clean the airbrush between colors since the idea is to make things look dirty anyway. This technique works best for really heavy industrial buildings like powerplants or steel mills. For chalks I grind a chalk stick onto sandpaper to make a powder and then rub it into the structure’s walls and roofs with a small paintbrush of the 50 cent use-it-once and discard variety. Hard rubbing and blending with progressively darker colors gives me a good effect. I don’t usually dullcoat unless I want a subtle effect because dullcoat makes most of the chalk weathering vanish.
I use a variety of water based colors staying mostly in earth tone colors to represent the area I am trying to model. If I’m looking for a rusty look on something I’m more likey to use the color “Georgia Clay” as I am the color “rust” in my opinion it gives it a softer look. I use the dry brush system with the attitude that tomorrow is a new day so I can always add more if I don’t like the initial look. When I’m done I spray it with Dullcoat.