Yes, the xerox technique has been using way before 
1990, and it has been using in every anime studio
nowaday.  The hand-painted ink lines can easily
fade out too if the cels are not treated properly,
and most importantly, the ink pens that the 
studios use.  Same thing to the xerox machines,
there are different quality of inks.  The high
quality ink accompanies with good xerox machine 
can create lines that can last a long time, and
looks like hand-painted lines.  Bottom line is
the type of ink that the pen or the xerox machine 
use can make a difference.