One
of the great things about this paint is that it can be diluted as much as
needed using Isopropyl alcohol -- although I have not made dirty windows, I
have done a lot of other types of dirtying up. I would suggest starting with a
very diluted paint (like 5% paint, 95% alcohol) in your airbrush, hit the
windows with a layer -- there will be a nice mist coming from your airbrush at
this ratio, let the alcohol dry and see if its dirty enough.  With this
paint, you can dilute it to spray a thin film of color or apply enough passes
to get a completely opaque finish.