Looks great. Not sure which airbrush you're using but if you turn the airflow down and widen your spray, you'll be able to soften the edges of the (in this case) black. This will make it look even better than it does. Don't worry about the soft (overspray) edge following the contour of the masking perfectly. In fact, a slightly irregular soft edge will add some realism. I'm not a stock painter but I made a living as an artist for almost 20 years and airbrush was one of my mediums.
I wish I had the money to buy all the equipment to prep stocks because I've always wanted to use my painting skills on them. I'm jealous! Lol! Keep it up - you're doing great!
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