Painting the underside of a swim platform?

While I have the boat out (again), I'm contemplating painting the underside of the swim platform. Its looking pretty Sh***y. I'm not sure what type of coating was there originally, but I hit it with a sander today, and a lot of it was loose and flake off, while some of it was well adhered

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How To Add A Swim Platform And Ladder To Your Boat

extended swimplatform

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Moisture in Swim Platform- Fiberglass Help Please - The Hull Truth - Boating and Fishing Forum

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Conall's Boat Build: Aft deck and swim platform

Conall's Boat Build: Aft deck and swim platform

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Original Oil Painting Downloadable Print (No Physical Item, Only Digital Files), Get this beautiful masterpiece to decorate your home or office.

Swimming, Contemporary Impressionism, Downloadable Original Oil Painting, No Canvas, Only Digital File!