[ecomodded]

If you cut a piece of plastic or wood or metal to the general shape of the curves of the boat tail, say
3 or 4 slightly different angled sizes to allow you to smooth the freshly applied *what ever* more consistently.
Once you get the form to shape I would throw some fiberglass matting with fiberglass over it then cover it in with filler/ bondo and belt sand the bumps out with varying degrees of grits, the 45 grit takes fiberglass off stupid fast, you can shape fast and effectively using a belt sander and low grit belts.

I made a large fiberglass deck roof for my 25ft. Bayliner, back when i owned it.
other then that i have done just repairs and patches, low grit sandpaper on a belt sander is the answer.