With a 1966 boat, I''m suspicious that perhaps you and the previous 12 owners aren''t compounding through the gelcoat. It''s not even 1/4" thick to begin with. The boat may also have been awlgripped at some point, and you may be doing (compounding) what awlgrip tells you in all their literature to NEVER do to their finish. We had a 1969 Soling that seemed to do well with a chemical polish made for fiberglass cars that I got at the local automotive parts place. It stayed shiny for the 25 years we owned the boat, anyway. Perhaps chemistry is the way to go, rather than physically grinding the surface with compound. Worth a try, anyway. Good luck.