I gave up using that stippling technique. Put down the airbrush and pick up an old toothbrush. Dip the tips of the bristles and flick with your thumb. I start with a solid gray, throw on some black flecks, some white flecks, do some cracks in black with white highlights, and Bob's your uncle.
Yes, that also works......I just learned with popsicle sticks and clothes pins.....there are many ways to get the job done...Thank you for your comments!