For a while, I was using acid-free 
archival quality glassine to back my cels 
(especially my extra-oversized cels). The glassine 
will eventually stick to the cel. However, it 
unsticks really easily, and I have yet to 
experience any paint loss when removing the 
glassine. I no longer back any cel with glassine 
because I don't like that it sticks at all. Any 
cel that fits in a cel bag gets stored in one. The 
oversized ones just get a cel bag positioned 
behind the painted areas.

Queenie