I wait until the "internal skid plates" start to get ground down and then build them up again with strips of fiberglass and resin. It's inexpensive, the glass is transparent (so your boat looks good), and when the resin sets you can sand and buff to match the original lines of the boat (thus not losing hull efficiency). It's an easy, inexpensive repair that you can repeat whenever necessary without compromising the sleek shape of the hull.