This is an section from a poster made back in the 1920s or 1930s, that was made using an airbrush:
What interests me most is that it has a rough, spattered quality. It's as if the airbrush didn't work very well and sprayed unevenly, resulting in varying dot sizes where the paint landed.
I've tried to get this kind of effect with the airbrush tool in Photoshop, by manipulating scattering levels and trying various brushes. However, I can't quite get it.
However, when I've searched for Photoshop tutorials, anything under the term "spatter" is way more exaggerated than I need, and anything with a "vintage" or "grunge" level is about the overall style, not specific brush settings. At least, not that I've found.
How can I set my airbrush to get a gradient quality like the example picture, or something similarly uneven in quality?
This is a section of a poster from the movie The Rocketeer, which has a similar effect: