I just started using plastic foundation last year and haven't had to
scrape any of it off yet, so I can't answer your question.  But you
describe a problem that has come up in the Santa Clara Bee Guild
meetings this winter.  Lots of urban beekeepers in this part of
California seem to have similar congestion problems.  Part of the
problem is that the bees are prone to scatter brood in pockets around
the brood box with patches of honey and pollen interspersed.
 
I'm not sure I understand why you scrape the comb off the Permadent.  Is
it just to clear out the congestion?  If it's mostly full of honey it
seems to me like you could just extract it and save all the hard work
the bees went through to draw it out.  If it is mixed with brood, you
could move it above the excluder until it's clear.
 
I'm in Gilroy, (40 miles south of San Mateo) and not so much in the
Eucalyptus. My 'country' bees slow down more in the winter than my
'city' bees.  I deal with the congestion problem by swapping out the
congested frames for extracted ones or foundation in extreme cases.  I
generally try to move comb that's clogged with pollen to the outside
edges and leave it in the brood box, or save it for a new colony.  I use
two brood boxes, which is a bit unusual in this area.
 
Michael Reddell
http://www.hotcity.com/~mwr