After attaching the vertical stabilizer fiberglass fairing, I filled that joint with foam and after sanding to shape, covered it with a thin layer of fiberglass.
phintz have you watched the installation video? It is not airplane specific, but Definately worth watching. Also probably not soon enough but there will be a two day training course in November for installing PolyFiber. Go here for more info and scroll to bottom. https://spenceraircraft.com/events/