Whether you're going to paint it or not, you'll want to remove as much oxidation as possible. If there is enough colorant left in the fiberglass or gelcoat skin, an anti-oxidant will bring it back to new, no excessive buffing is needed.
I like to use phosphoric acid based products. Water softener liquid cleaner is a cheap source. Body shops that sell Martin-Senour products sell a series of etchant cleaners/deoxidizers (used to be Iron Etch I, II, and III). The latter two contained surfactants to clean the surface. If you have Dawn, it will suffice. The etchants are sprayed onto the surface and either brushed or left to sit on the surface until oxidation is removed. I'd suggest to brush lightly with a soft brush on the first coat. Reapply a second coat and immediately rinse. The oxidized color should run off onto the ground with your applications and rinse. When dry, follow with a polish and then wax with a marine wax. Done right, the above may get you back to "new". Good luck.