Although my hulls are painted with a urethane light blue, I have a black (racing stripe) on the bottom edge of them to protect them from the sand. I used layers of fiberglass cloth and resin similar to: "The Bottom Job - This Old Hobie" by Rick Buchanan on pages 12 & 13 of the Sept/Oct 2008 Hotline
Instead of putting on a final coat of gelcoat, I used more resin with West System 423 graphite powder. It helps the hulls to slide better over the sand and gives a shiny black protection strip.
If you ever painted your hulls with any color other than the original gelcoat white, you will be able to easily tell where you need to protect the bottoms of the hulls (and paint) from the sandy beaches.