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Old 06-06-2013, 12:27 PM
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Sodar Sodar is offline
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Join Date: Nov 2004
Boat: '07 ProStar 197 w/ MCX
Location: Seal Beach, CA
Posts: 11,060
It is actually mold/mildew/organism that has gotten into the vinyl. When the cover or plastic is put over the vinyl, it does not let the vinyl breathe, the vinyl bakes in the sun and it turns black. This is why the hot sun and air takes the stains away when the cover is off. Nothing is going to "clean" the stains off. This typically happens to boats that are stored outside in the southern states. Humidity exacerbates the issue, so drying the boat out as much as possible before covering will help.
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