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I had some equipment cleaned with a dustless blaster a few years ago. It is just as aggressive, maybe even more so, than regular sand blasting. The water just eliminates the dust in the air. I would sand it with a "double action" rotary sander like the auto body shops use. If you frequently change paper, it will go much faster than you think it will. You will have to sand it anyway, even if you do blast it. I would not risk damaging the gelcoat. Sanding is one of those jobs that is dreaded until you do it, and then you wonder what the big deal was.