Controlling fine line intensity with an airbrush

I've tried to airbrush freehand fur before, which wasn't really successful. The reference image wasn't very good (printed on an old HP 1010 B&W laser printer) as a result of which I wasn't able to make good use of my lightbox. I am currently working on an other project for which I had the reference image printed in a print shop. The image that I found on Internet, was not high res, but good enough to see most detail. The beard texture in the recent project is not entirely similar to the fur texture of the panther's fur, but it requires a similar technique.