Iridescent paints are tricky. While the binding agent in them is already pretty watered down (they seem thin to me) the particles are at times just a tiny bit big for the brush. I have a Paasche gravity feed airbrush and the colors go through fine for me, I have to watch for drips myself. What sort of airbrush are you using? You mentioned the different needle sizes, are you changing the tips for each size as well?