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I will start this off by stating that this article is completely my opinion and what I have learned and figured out over time in my shop, and through lots of research.
I use primarily Liquitex Softbody Acrylics in my studio. I do this because Liquitex makes paints with very good pigment load, lightfastness, flexibility, and durability. I can also thin them with airbrush medium and use them in my airbrush. The other paints I use in my studio a lot are acrylic inks, either Liquitex acrylic inks or FW-inks. Once in a while I still use inexpensive craft acrylics but only as a primer or undercoat or on a test piece, or concept, or when I need a very specific color and am lazy, too lazy to mix it up. I can also use the softbody acrylic when doing brushwork, so I only need to stock one kind of paint.
Terms
Pigment load: Acrylic paints consist of acrylic polymer emulsion (binder) and pigment. The ratio between binder and pigment is generally called pigment load. The more pigment in relation to binder the higher the pigment load.
Lightfastness: The ability of the paint/pigment to remain unchanged after prolonged exposure to light.
Thinning paints for airbrushing.
Types of thinners
Airbrush medium: A mixture of acrylic emulsion (clear acrylic paint, acrylic polymer), and flow aid (reduces surface tension, reduces tip dry). A good choice for most applications.
Distilled water: H2o. Reduces pigment concentration and color strength. Depending on substrate and amount of thinning required may be suitable for some projects.
My thoughts on thinning paints:
Acrylic paints are made up of an acrylic polymer emulsion(clear acrylic paint) and pigment. When airbrushing with acrylics you need to thin them somehow. If you were to add just water to them you end up stretching the acrylic emulsion to much. If you stretch the emulsion to much the acrylic paint has a tendency to crack and peel over time after it dries. If you think about it, what happens when you airbrush is you are spraying out tiny little droplets of paint onto the surface of your artwork. Water has lots of surface tension, if you have a lot of surface tension in those droplets of paint you get spatter. Spatter will cause runs on the surface of your artwork. After all you have a water trap on your airbrush, why would you thin your paints with water? You could use alcohol it has low surface tension. Some airbrush artists use windshield wiper fluid to thin acrylic paints. I have a hard time recommending this. Windshield wiper fluid contains large amounts of ammonia and alcohol these will evaporate, what you end up with is a very thin layer of acrylic emulsion that is stretched too thin.
What you need to thin your acrylic paints with is something designed for this purpose, airbrush medium. What airbrush medium contains is acrylic emulsion and flow aid. Flow aid reduces surface tension which reduces spatter and airbrush clogging and also retards drying time, which keeps the tip of your airbrush needle wet during use.
While I could just buy acrylics that are already made for airbrushing, I would then have to buy 2 kinds of paints for my studio/shop. There is a benefit to this of course, I would remove some of the frustration I encounter on occasion when I don’t add enough paint or medium and clog up my airbrush, or make the paint too thin.
