My new compressor puts out less pressure (15 max), so I get even closer than before, and find I am about two inches from the surface to be painted. I consider an airbrush to be more like a regular brush, not a spray gun. And, yes, like the previous post, I find grainy finish usually results from too great a distance, too high a pressure, or both.
BTW, what is Testors Opaque. Is that a color, or a new type of paint? I had not heard of it before.