Mike,
I'm honestly not sure about the binder statement; I know that binders help stick the pigments to each other and to the substrate, but you still have to get it from point A to point B through some sort of solvent vehicle. In my experience, simply because a manufacturer says their paints are non-flammable (ie,Testor) doesn't mean they won't burn. Flammability and combustibility are based on "flash point"--the lowest temperature at which a material will produce enough vapor that in the presence of some ignition source it will flash (not burn). Flammable materials have flash points less than 73 degrees F, while combustibles are greater than 73. There are also some boiling point considerations, but for this discussion, temps will work just fine. So, out of the bottle (like for brush painting), acrylics are safer than enamels, which are indeed flammable. HOWEVER, once acrylics are thinned with, say, isopropyl alcohol (a flammable solvent) and put into an aerosolized mist through the process of air brushing or rattle cans, acrylics become quite flammable. The big difference between enamels and acrylics now is the vapor concentration in air and it's relative degree of evaporation (volatility): it should take more acrylic vapors to flash than enamel vapors. Unfortunately, we have no way of measuring (at least economically) vapor concentrations in each and every spray booth, so we need to take the same precautions regardless of the materials we use (brushless fans, adequate airflow and ducting, correct construction materials, etc.).
But what if I'm just using water to thin my acrylics prior to spraying? Then the hazard is lessened substantially, and the only concern would be the type, concentration, volatility and flash point of the base solvents. Of course, these most likely would vary somewhat between manufacturers. But now you're in the area of the unknown, so once again, the same precautionary measures (as above) are taken to prevent the unexpected uh-ohs.
One last thing, and I'll shut up. Rattle can spraying is a little different than setting up an airbrush. First, solvent concentrations are considerably higher, since higher volumes of paint are used over the same time. Second, they are already pre-thinned to get the mixture through the nozzle, and third, the addition of propellants increase the product's flammability. Manufacturers (at least the ones I've seen) typically use isobutane and propane as their primary propellants, although I have seen some carbon dioxide used, but only sporadically. Conclusion? Spraying with rattle cans is considerably more hazardous than airbrush painting, even with acrylics, given propellant considerations.
Sorry this was so long, but I hope this helps, Mike. If you have any questions, or if you simply think I'm full of it, shoot me an e-mail.
Gip Winecoff
PS This is all taken from a fire hazard standpoint. Stay tuned next week when we expose Barney to 500 ppm of methyl ethyl death and see if he turns from purple to green! (Health hazards are a little different animal.)
GW