well i must say that i use airbrusher for my scenery. I needed to create smooth differences - or depictings?- on my colour schemes.
For instance, in the Generator i went from the darkest grey to the lightest - i used some Vallejo paints putty ( the blue one) that is applied before airbrusshing, and then retired, once dried ( that gives the paint layer chiping effects).
Also, there is the weathering part ( the funniest!) where you can use oil paintings, inks, chipping....salt techniques and the like.