Cels Forum

Besides using acid free material, you should also consider
using the glass with UV protection.  Also, avoid placing
your cels in humid and hot locations.
Another reason of the fading is the ink (not cel vinyl
arcylic) itself.  Different studios use different brand 
names of ink for either hand-painted or xerox.  Some brand 
names are not as good as the others, like some of them are 
quite diluted, and they tend to fade out faster within a 
period of time.   For example, I noticed the blank outlines
of my Sailor Moon and Dragonball cels tend to fade out
a lot faster than my other cels, I wonder Toei Studio uses
a cheaper brand of ink.