I do not believe wax paper is acid free. If you 
are going to back your cels, I would advice using 
acid free tissue paper (archival tissue paper is 
what the art store call it in Birmingham). It is 
easy to tear away and you do not need to worry if 
it sticks to the cel or not. Search the forum for 
previous discussion on this issue. This tread has 
been brought up multiple times.

Marcus