Actually, info on fixing line fading has been
archived in the "Okay 
could I get a couple of opinions..." thread 
and "I 
finally got the lines cel done :)"

A rapidograph apparently works nicely; common 
transparencies also work for the correction layer
itself... And it's true that UV isn't the only 
culprit; I still think it's best to store cels in 
cel bags, then cel books, then a dark shelf or 
closed box. ^_^' Humidy and temperature probably 
also play a factor...
Good luck