If mixed use means some salt water and if it has scrapes, then time to paint the bottom. I do not think the bottom paint is waterproof, but I have not been in salt water for almost 20 years and a lot of development in paints has happened in the last 20 years.
My concern is water seeping through the scraps causing more trouble. You might want to address the scraps before you paint, at least talk with someone who knows fibreglass and morden bottom paints. The ocean marinas should be able to help here.
Good luck with it.
R2B
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