An air brush is an amazing tool, but is hard to perfect. If you decide to take the plunge again I would build a few "basic" models to practice on. You could always just use the grey spray, wash various shades of blacks and light dry brush.
For dirt and rust its easy. Just grind up oranges and brown artist pastels. Then fix them with some hairspray. Just google weathering powders for tanks and what not an you will be inundated with tutorials. The key to powders is putting them on a gloss varnished base....
Well, thats my two cents. Keep up the good work!