When I paint, I usually prep with 320, then 400 then hit the entire ski with 3m scotchbright (wet) as a final touch. It is overkill, but the better you prep a ski, the better it will look after paint. Also, those marine epoxies are very durable, but they aren't going to give you a real high quality finish. (if that is even what you are looking for) If you are going simply white or black, those are fine, if you want a candy or a pearl or just want something that looks showcar when done, your best bet is base clear with quality paint from a paint supply shop. Just FYI
SM