I would agree with what everyone said so far. You 
shouldn't need to put anything between the two 
layers. Of course it doesn't hurt. Just be sure 
there are no areas of paint on the correction 
layer. I have a few cels with correction layers 
and all but one have some area of paint. 
Sometimes they recopy the outlines and then paint 
in only the one small area that was done in the 
wrong color. In one of mine, it is rather hard to 
see the painted area since it is a dark grey and 
blends in with the outlines very well.