Thinning ratios for airbrushing is a constantly reoccurring question and there is no right answer. The most common answer is “thin the paint to the consistency of milk.” . . . Right. . . Whole milk? Skim milk? Buttermilk? . . . You get the idea.
The paint manufacturers offer their own suggestions, but these are just starting points. How much air pressure you’re using to spray will also come into play: the thinner the paint, the lower air pressure you need. Andy Bloom put together this handy list which can be used as a starting point.