Thanks for the info. Super useful. I will do some experimentation as well.
Here is a reference image: https://www.dropbox.com/s/i0u05jorae6yba8/Screenshot%202018-03-27%2012.36.52.png?dl=0
The yellow lines are putty along panel lines. I was thinking of masking off the panels themselves to leave just the putty lines exposed, then airbrushing (after the base NMF is down.) The lines need to be sharp but not straight, and I've seen this done elsewhere so figured it'd be a good method.