gel coat like everything else will wear with abrasion and in some spots may have originally been thinner than others. It is also damaged by UV from the sun. If there's a thin patch in a high traffic area such as the deck, it can simply wear through.
I think the solution is called "paint". You can also apply more gel coat, but it's more expensive and more difficult; and I'm not sure the results will be any better.
I think you'll understand the value of a good paint job
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Willie
She's a tired old barge but she's paid for!
http://s71.photobucket.com/alb...p;current=ef324993.pbw