This article is from the Miniatures Painting FAQ, by Brenda Klein email@example.com with numerous contributions by others.
The best paints for airbrushing are Accuflex and Humbrol, with Polly S
and Testors each selling an airbrush thinner for their paints.
That's the bulk of what the author knows on the subject.
Some excellent information was posted to rec.games.miniatures by
Mike N. Tassano (firstname.lastname@example.org), much in regard to advising
a novice airbrush painter, and is reproduced here without permission
and with minor editing:
"I've done a lot of airbrush as well as regular airgun painting, so
maybe I can get you pointed in the right direction.
"There is a relationship between the airpressure used and the rate at
which the thinner evaporates. Ideally, the carrier or thinner is still
liquid when the paint strikes the surface to be coated, but not so
liquid it runs off. Inks have a really slow thinner, relatively, but
since you're doing a wash, you don't care if it's really wet on
contact. The idea is to puddle ink in the low spots anyway.
"The primers usually have a fast thinner, allowing a good coating
without running. Spray cans "usually" are balanced between pressure
and range and thinner and particle size.
"Second, the pressure in the air-cans varies wildly as you use it up.
And as the temperature changes. (So does the moisture content from
condensation caused by cold air) Even the best airbrush will behave
in a cranky way with canned air.
"Third, the type of paint or ink used may not be too friendly to
airbrushing. Particle size needs to be pretty consistent for spraying.
A lot more consistent than brushing requires. If you intend to stay
with airbrush priming, I can offer some possible helps:
"1. If you can ONLY use canned air, shoot for shorter sessions. Let
the can warm back up a little more.
"2. Try an alternate air source, a compressor or an innertube filled
at a service station. You want as little pressure difference between
your air source and the spraying pressure as you can manage.
"3. Use a primer designed for spraying. There are some hobbyist
brands around that might be available where you are.
"4. Practice, practice, practice!"
And a word about priming, thinning and cleaning from
Ed Sharpe (email@example.com), which is also edited and used without
"After carefully cleaning, washing and drying the figures, I prime
them with Testor's flat white mixed 50/50 with airbrush thinner by
Testors. I apply the paint using an air brush. It usally takes 2
to 4 coats. Take your time and do not rush any of the steps. I use
the Testor's air brush thinner only to thin the paint. I use general
paint thinner from the hardware store to clean my air brush."