I have received a lot of cels that are stuck to 
the sketch. This is what I do and it works great 
for me. Check around the cel and find where it is 
stuck the least. With your finger nails, palm 
down rub ever so slightly around the area that is 
stuck. Make sure to rub around the whole image as 
much as you can. By doing this it frees the paint 
up all around and in the middle. If you cannot 
find anywhere on the cel that you can't get your 
hand under I found that there are only two things 
left 1) Leave the cel & sketch Alone or 2) tear 
the sketch little by little literally to pieces 
until you can peel the sketch completely off of 
the cel, Yes, this way will destroy the sketch. 
But you will have the cel nice and clean looking. 
Hope This helps. Good Luck '_'