Cels are rarely hand-inked these days.  The black 
outlines you see are usually xerox'd onto the cel. 
 That's why they don't hold up as well to light, 
crappy cheap toner in copy machines that haven't 
been serviced in years and leave powdery looking 
lines if you look closely.
Older cels that are hand-inked have outlines that 
last a good deal longer than the machine inked 
lines.