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There is a primer you apply to the hull before you bottom paint. Gel Coat is not 100% waterproof. I used Petit Protect Epoxy Primer (comes white or grey) made specifically for gel coat application. The paint and primer follows the waterline. Mine (done by others before the purchase) is just above the water line but you should get it as close as possible and try to be as exact. One side is a little taller on my boat and I thought the boat was listing to one side.
Garris