Sorry to jump in on your thread. I've been meanig 
to ask that for some time and you beat me to it. 
I also have a cel with a significant amount of 
line fading. I love the scene and the character 
so much I really don't care, but it would be nice 
to have a correction layer. Here's the hard part. 
It didn't come with a sketch and is not from a 
popular series. It will be very difficult to even 
know where the lines are supposed to go in some 
places (VERY faded). As suggested above I have 
included a scan of the cel. Please anyone contact 
me if you think you can help. It really is beyond 
my artistic skill to attempt personally.