Hello. I have same issue around belly band with my Scamp
. Yup it's mold and mildew. Concrobium from home 0epot will get rid of it! Mix it according to directions and spray on with pump sprayer. Leave on 4 hours or so out of direct sun then rinse. Does a great job and does not harm fiberglass or Gelcoat. The spots do come back eventually. In my case, i do this once or twice a season.
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