I put my cels in cel bags with cardboard to 
prevent bending b4 inserting them into cel book 
sleeves. I think protection of cels is more 
important than how they look temporarily under all 
that plastic. Personally, I would advise against 
framing unless u live in total darkness 'coz light 
is a real enemy of any artwork. U might not notice 
the dulling/fading of colors until much 
later....then of course it'll be too late.

I also bought those paper frames(mine's black) 
that are precut to frame the cels.....I bought 
them in Japan but I'm not sure if anyone sells it 
online.By taping the cel with the cel bag onto the 
frames, it'll prevent any sliding around since the 
frames are all standard B4 size. And it looks 
really nice too!

BTW do remember to store douga & cel 
separately to prevent adhesion.....Come to think 
of it, I'm not sure if my cels will stick to the 
cardboard....and sometimes I worry if the BGs get 
stuck too.... if I use acid-free boards, will 
sticking occur?