Anyway, I have sanded the black back now, and it's pretty good, though not perfect.
I don't know I have a "perfect" finish job in me. I've only been spraying with a gun for couple of years (3 or 4 guitars-worth) and whilst I have gotten more confident, I've yet to achieve perfection. However I've been vastly more satisfied with the results than I used to get with rattle cans.

...I have also made a binding scraper...
Did you do the split wooden dowel & bolt trick???