Windex, after time, will actually DULL (grey) 
acetate because it has **ammonia** in it.  Its a 
big fat no no for cels.  You Brillianize if you 
want, because it was designed for plastics.... or 
as in the below post, you can buy special 
cleansers.  Trust me, if you want your cels for a 
long time, stay away from Windex ;)