Get some bondo or whatever you like to use for filler. The six panel hull for your new coffee table...er...C57-D is supposed to be smooth, so you're going to have to fill in all the seams.
I never built mine. After building two PL Jupiter 2s, I decided to sell my C57-D. I just couldn't bring myself to go through the trouble of polishing another smooth hull. It's amazing how many scratches that you can't see show up after the first coat of metalizer spray paint!! Get some sandable primer and a lot of wet sanding paper/pads for getting out all of the tiny scratches.
BTW, this would be an awesome project to try out the Alclad paints.
Raygun