Layers of paint

In principle this works with brush painting also. You may have noticed how you'll get streaks in your paint when brushing? Well, if you thin the paint it will level out better but you need three or four coats to get the color and kill the streaks.

When airbrushing you will get better results if you thin your paint then apply one light coat to get a 'bite' on the model. Once that has dried apply another light coat to start getting color density. Let that dry then apply a third, fourth or even fifth coat to achieve the color density that pleases you. This allows you to have a good degree of control over the balance between color density and paint thickness - don't want to hide detail under thick coats of paint. It also helps to prevent runs, which are a pain to fix.