Technically, I think the problem is the line copy 
machines that the studios used.  It's similiar to 
the regular copy machines that we are using, but 
they use specifically on acetete instead of 
paper.  Different copy machines produce different 
quality of lines, as well as how long it can 
last.  Also, the toner quality makes a difference 
too.  I have some cels from 15 years ago, the 
line quality is still good, but I have some other 
ones from a few years ago, the lines have been 
fading or the black powered substances that 
produce the lines are starting to fall off.