Jim
Can't speak for Super Fil, but I did what you are contemplating on my first Model IV. I was then pretty heavy into Epoxy so used Epoxy with microballoons as a filler to make up the paste. Down side, it sets pretty slow, but it stuck fine. My plan back then was to prep one of the fairings and use it as a plug mold for fiberglass fairings. I got some bad advice that scared me away from that idea and put the PVC on with the grooves sanded off and any sanding glitches were prepped with the micro. I think Super Fil or Featherfil are more like Bondo using polyester resin, but with the light weight filler. I would think they would work especially the prep sanding.
Lowell