Messy style of brush you're using.
Use layers so you can paint over sketch + remove its outline later. Mapping is good. Use reference for shading.
Maybe you would like this style:
I am rather busy right now, but that is such a great outline for a head that I gotta texture it.
So expect a free edit here in the next few days.
I plan to sharpen the lines to 1-2 pixels thick, give it a coat of digital paint, then scuff and mar that paint, finally I will shade it into a bit of depth.
If you would like me to post pics as I go and give a short tutorial with em, lemme know.
..