I have attached a pic below of a cel with MAJOR 
line fading.  ;_;  (it is currently in transit)  
It absolutely broke my heart to see this wishlist 
cel in much poor condition.

My guess is that the previous owner may have had 
this cel on display where there was a lot of 
light.  Light is the #1 culprit of line fading.  
(natural or articicial light)

How can it be avoided?  One way is to keep your 
cels in an archival safe book away from windows 
or direct light of any kind.  If you REALLY want 
to display your cels, make sure that they are 
framed with UV protectant glass or UV protectant 
plexiglass (which isn't as "clear" and tends to 
be easier to scratch).

Even with the UV protection, make sure that 
framed cels are exposed to as little light as 
possible as that it can still fade lines 
gradually over time.

Hopefully the scan will show up below. (crosses 
fingers)

jINx