Bill,
   Use tape for your parting line.  It leaves nearly no gap to start.  Hand
sand untill you can fit folded sandpaper into the gap when panel/door is
closed.  Gradually sand it out untill you are using stiff cardboard with
sticky back sandpaper on both sides.  Make that tool as thick as you want
your gap by adding paper.  It should be sanding both sides of the gap.  Your
gaps will be perfect.
Walter Dodson  IV-P in test 13.5 hrs