Well, sadly most anime cels are not made to last 
beyond production.  The Japanese use Xeroxing to 
make those trace lines, so, eventually those 
lines are going to fade, it's just a standard 
copier making them.  I have some production Bluth 
cels that have been framed for years, and there's 
no damage at all...but, those lines were done the 
old way, using paint and real ink.  These xeroxed 
lines on almost all anime cels are going to fade, 
even if you keep the cel in comlete darkness.  I 
think it's pretty sad to know that even your most 
precious anime cel will eventually lose value 
over time due to the lines.  But, the truth is, 
the studios dont make these cels with the 
intention of having them last more than the time 
it takes to run it through the camera. Pooh.  And 
I have so many cels I wanted framed ~_~.