Air brushing paint and enamels onto glass can give extremely subtle graduations of colour and tone.
The consistency of the mix of the paint or enamel with the medium will need to be many times thinner than that used for painting with a brush. Also the air brush will need to be able to cope with the relatively large (in relation to inks and other paint) particles that make up the glass paint. This may require a little experimentation to find nozzle sizes that can cope with the glass paint particles.
There are two main media that you can use – there are others of course. Water and alcohol or methylated spirits are common and easily available. The advantage of spirits is that it evaporates from the surface more quickly. You do not need to use any gum arabic in the mix to help it flow. You could however add a touch of washing up liquid to overcome any surface tension within the mix.
The paint should be applied in steady sweeps across the area to be covered about 300mm from the surface. Start moving your brush before you switch on the paint and keep moving after you stop applying the paint. This avoids heavy applications at the start and finish. As there is no absorption of the moisture by the glass (as there would be on paper or board), you must pause after a very few passes. This is where the spirits show their value, as they evaporate more quickly than water, allowing you to apply the next layer sooner.
You can assist the drying by using a hair drier to gently blow warm air over the surface. This will reduce the waiting time between applications.
One thing you will notice is that the paint will settle within the medium unless you agitate it frequently. So you should make sure the paint is evenly dispersed within the container by agitating it before starting each layer. The movement of the air brush during application will be enough to keep the paint suspended in the medium while you are applying the glass paint or enamels.