I have done a lot of printing on 4-ply museum board and am familiar
with the problem you are encountering. Assuming you are working for a
client who is not interested in being educated on the finer points of
beat fat/beat lean (although it's hard to imagine such a person),
here are some suggestions:
1) Remove all the packing from the tympan, pull an impression and see
if you are still getting a plate impression.
1a) If you still see the plate without any packing, you will need to
back off the platen prior to proceeding with the steps below.
2) If without any packing you are NOT getting a plate impression,
progressively build up the packing until you start to see the
impression. Once you do reduce the packing by 2 mil.
3) Trace an outline of your plate onto a sheet of packing material (I
usually use 6 mil tympan paper) and cut it out just inside the line
(ie. the packing should be slightly smaller than your plate so that
the plate edge does not contact the paper).
4) Due to possible shift in the packing, I would mount this packing
directly onto the tympan using either a spray adhesive, or carefully
positioned dots of pva. You can usually increase the packing within
the perimeter of the plate by at least 8 mil before you will
encounter the next problem which will be getting an impression of the