I've done a couple of boat hulls and several outdrives, other odds and ends using spi clear... definitely 320 for adhesion - AFTER thoroughly cleaning and then wetsanding the hull until the oxidized layers of gelcoat are gone (oxidized gelcoat is like a sponge and will hold all sorts of impurities, i've typically wound up doing this with slowly running water using 32o on a DA for clear gelcoat). if there's any metalflake or color you want to fix, now is the time to do it with gelcoat repair. I've had really good results with big chips in flake using a dremel tool to grind out the area, mixing a flake with the same color and size as the hull has, filling and filling the area with that. then you can use an airbrush to carefully "correct" any place where you had to remove a color border using when repairing the gel / flake... I've just used a little basecoat that matches the boat color. If any time between sanding and painting, I have scuffed (avoiding anyplace where I've brushed in / hidden a repair), cleaned thoroughly with por15 marine clean (just what I've had experience with) clear water rinse and shoot the clear. For any "regular" auto paint, only use below waterline for a trailer boat, and generally the euro clear is harder and more durable vs abrasion.... That's just my experience... my ski boat looks great still after 5+ years of hard use on the river here...