Wait a sec, I'm not the one with it written on his shirt that'd be you Mr.Alpha!
Ed, Chalk and a nice big brush are great because you can apply as much as you want till you get the look you want and if you mess up just wash it off and start over! Once you get the look you want you spray on clearcoat to seal the chalk making it permanent. Arts and crafts stores sell nice sets that have a wide variety of colors usually light yellow to very dark red will be in one pack, where white to black will be another, same for green colors from very light to almost black. I only ever use the yellow to very dark red pack as it does all the colors of dust and dirt very well.
Take the chalk and rub it on sandpaper, once you get a pile of chalk dust take your brush and dab it bristles straight down into the chalk dust. Then just brush the chalk on the model where you want/think the dirt and dust would be.
...there's a cool video on youtube...will post it if I find it...put it here before...a girl was weathering tanks...great job of it too....using soft make up brushes
