Could I just bond to the 'toothy' surface created by the
 removed 'peel-ply without sanding it? Am I getting a better or worse
joint
 by sanding it?
 
A composites guy I know who did quite a bit of testing on this has a
rule of thumb: "If you can sand it - sand it".  By that he means that if
the epoxy is still tacky and cannot be sanded, then lay up on it.  But
if the epoxy has cured hard enough to sand then do so.  Seems to work. Robert M. Simon
ES-P N301ES