hello , and I have a 1972 21 footer Globetrotter . I have used 800 3m paper
on a 6 inch double action air powered sander , But 1000 grit would also be a good choice .
Sand it slowly and carefully to make a uniform gray .Then I would hit it with
an orbital wool buffer pad at about 600-800 rpm with any good compound .
then finish with Mothers with a orbital buffer or Nuvite
I have done alot of major scratches and have had zero problem with this method .
Take your time , get a beer , and enjoy the results !!!