Cels Forum

I'm tending to agree that you probably should look into
some conservatory style glass.  It's usually quite expensive
but I have it on all my framed cels.  It gives better UV
protection than standard glass.

There is another thing that might be occuring.  Paint does
give off fumes and I've been told that you want to take the
back off from time to time to release the built up gas.

So far I haven't tried doing this with any of my cels. But 
it does kind of make sense that releasing the fumes could
be a good idea.