Go to where they sell auto body shop supplies. Buy a box of Body Putty.
The old time bodymen called it "dumbo," I believe. It comes in a box, and in strips, with wax paper between the layers. They stick it in body seams, and then paint over it. It doesn't harden, and then it is easy to remove. Put your wax/ release agent over it. It comes in two colors: black or white. It is sticky, and a bit stiff. It reminds me of another product called Mortite.
As a side bennifit it works good as a broken case extractor for a headless case stuck in a rifle's chamber.
