Open water reflects and magnifies ultraviolet, so the hull sides and transom take far more UV load than the same surface would on land. It is the main reason gelcoat degrades as fast as it does on a boat that only gets used at weekends.
When salt water dries it leaves microscopic crystals on the surface. Those crystals pull moisture back out of the air and hold it against the gelcoat, so the damage continues on the trailer, in the driveway, days after the boat came out of the water.
The end state of untreated gelcoat: colour gone flat, white surfaces gone yellow, and a powdery residue on your hand when you touch it. Caught before the gelcoat goes porous it cuts back and seals. Past that point it needs painting.
We work topsides, transom, deck and superstructure. Anything below the waterline is antifoul, and antifoul is a specialist job with completely different requirements - we do not touch it and would not recommend anyone coating over it.
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