Happens to all boats old and new here in FL. Its more than likely not porous gel coat just high mineral contents in the water like iron or tannin from estuaries. LI sound is not the cleanest body of water either!! I use a very cheep product called "Pink Stuff" A whole 1 gallon jug is less than $10. You can find it at your local paint store. It is used by painters to remove rust prior to painting homes. I dilute it 50% with water. I always use a good polish on the hull after. Keeps the staining gone a bit longer
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