I bought a cel with the intention of doing just 
that.  Otherwise i would never have been able to 
afford a cel like that if it was perfect.  Still 
it was not cheap and i had some competition!  
There was barely a bit of black left on it!

My first attempt was not too good as my hand was 
not very steady as i was watching anime while i 
did it but second time round it was much better.

But i've seen cels which have additional layers 
on them with trace lines done by the studio.. but 
they are usually partial corrections.