Acrylic paint can be thinned for airbrushing by adding distilled water to the paint until it has the same consistency as milk. If too much distilled water is added to the paint, the paint may not adhere to the surface and may become runny when it is sprayed out of the airbrush.
Deciding how much an acrylic paint is thinned for use in an airbrush is dependent on the type of airbrush that is being used and the amount of air power that is behind the airbrush. Most airbrush companies will include instructions for thinning acrylic paint or will have a recommended listing of the type of acrylic paint that is best for the airbrush.
It is important for people who are using an airbrush to understand that airbrush acrylic paint is different from craft acrylic paint and most craft acrylic paint will not work well with an airbrush. The type of paint that is intended for crafts has larger pigment particles that can clog the airbrush. The airbrush paint has smaller particles of pigments that will flow more smoothly and will adhere better to the article that is being airbrushed. Most airbrush acrylic paints that are intended for use with a specific airbrush unit do not have to be thinned.