Hi Mj! Those are some gorgeous pots! The guys are probably right about having to waterproof the clay, but I would try a quick experiment first to see what the seepage rate really is. Thick walled clay pots like that might not seep nearly as much as regular ones. Plug the drain hole, fill them maybe half full with water and make a little mark with a pencil at the water line, then see how fast the water level falls. . Also in the sun for 6 hours per day, you're going to have a fairly high evaporation rate anyway. My little pond is in the sun only about 3 hours, and it can lose over an inch per week if it doesn't rain. Let's hope some day it won't rain??
I have a clay pot that someone painted with a dark colored paint and it sure seems to stick. In the ehow link Becky gave you above, I think the exterior varnish idea is the best, and it wouldn't discolor the clay too much either. I'd try some good marine varnish.
