Quoted from CaptainNeo:
...As far as doing it better than a human can. I don't see it being much different. Clear levels itself out. All comes down to how flat you have your playfield to begin with. Not that hard to keep uniform patterns when spraying. It's kind of like the auto industry. Factory paint jobs done by a robot, or hand paint jobs done in a shop. From what i've seen there. Hand paint jobs are far superior to factory robotic ones.
The quality of the robotic spray is determined by the end user. Try clearing 400 playfields in one day sometime and let's see how good your( or anyone's)work is.