If you're painting a cel and that happens--- the 
paint cracking and tearing--- you might check on 
the kind of acetate you're using. Does it have a 
coating on it that would prevent the paint from 
getting a good 'grip'?

That happened to me in the past, and I found that 
when I switched acetate, my paints took just fine.

To prevent the curling, you might want to let it 
dry at room temperature. ^_^ Perhaps the paint 
problem isn't with your acetate physically, but 
with the cold temperatures and curling. But the 
only way to find out is to experiment and see if 
it still cracks when it dries at room temperature.

Good luck!
-Cres