The plexi does need no pre-treatment. But it must be perfectly clean. Many other
plastics work, too, but I found the original plexi the safest in terms of
frilling.
<I am also wondering if developing separate C, M, and Y layers on 3 pieces of
plexiglass and then transferring one-by-one to the final paper will work?>
That's what the tricolor carbro and carbon workers usually do. To get a full
description read Luis' book. I learned it from him.
Klaus