IMO the spotted circles might look a bit better if you vary the distance between the airbrush, the mask (?) and the car body, and maybe hold the mask at an angle too. I won't need to tell you to play around with it, as you know far better than I do.
One other idea: My mother was a professional artist. Fine arts paintings, although she was not well known. She showed/sold paintings in about half a dozen galleries, and did a few cards for Leanin' Tree, almost all of them acrylics. And a few for the Audubon Society as well.
One of her techniques was to sprinkle fresh, but not yet dried paint with salt, and in her case the idea was to represent falling snow. You MIGHT try it yourself with a very fine--micro-fine--salt too, for the sake of weathering, rather than snow. Or perhaps even another reactive or absorbing "dust." Talcum powder? Flour?
I've been contemplating it too, and I think I would also try using very small micro-fine (I'm working in N scale myself, but it should work in HO too) open cell foam "sponges" on a stick.
I haven't tried it yet...so you go first....
And report back on all of your results with all of these, of course.
