Looks like a dry spot during lay up ! Grind/sand away all the loose gel coat ! Get a 1/2 pint can of gel coat, and hardener ! Fill it in, then add a second coat, and a third if needed, sand with consecutively finer sanding paper 180,200,250,300,400,600,800,1000 until you get to 1200 grit or so ! If it is smooth from new to old or the other way around, then polish with some rubbing compound, and wax !
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