I experienced the same problem, and the line 
fading occurred very quickly, so I do not think 
it is a matter of the protective coating wearing 
off the glass. Unfortunately I don’t think that 
any treatment to glass is strong enough to 
afford complete protection of UV damage. The 
only suggestion I have is what you don’t want to 
hear…to put them in completely dark storage.  I 
recommend scanning the cels and making a high 
quality print…then replace the real cels with 
the copy in the frame, and keep the original 
cels in dark storage.