I painted the bottom of my Great Auk 14 with a mix of epoxy, graphite and a touch of silica. This has worked out really well. It shows minor scratches, but has proved to be very durable. I would highly recommend it.
On another note, I recently refinished my Shearwater 17 and put epoxy and graphite on the bottom of the hull. Last weekend I thought I was going to paddle but my companion didn't have their dry suit and the fog was too thick. The boat was outside in the rain and cold for about 4 days. When I brought it inside before a snow storm the graphite had turned white and mottled. I'm not sure why this happened. The Auk, which had been outside temporarily in the snow, was fine.

Dan