John, experience, opinions, techniques, and stories vary on this - as on everything else in paddling and Life in general. I mix about 20% by volume of powdered graphite into the epoxy resin, then stir in the hardener. Small batches, one or two ounces at a time. I apply three coats, letting it set a day in between. (varies with temperature) I put it on from the waterline down, mask it off, and replace the tape for each coat, moving it back 1/8" each time to step the edge so I don't have a sharp edge.
My experience in sharp, broken rocks is that it does a fine job of protecting the boat. Some think this is becasue it is harder than just pure epoxy; others think it is because it is slipperier. Either way, it seems to protect. In looking at my boat after a trip into the Canadian Shield rocks (quartz, flint, granite, gneiss, etc.) I see scratches and gouges traveling up the side of the boat from the bottom. On the bottom (graphite area) it is a mere scuff mark, but up on regular epoxy it changes to a scratch or gouge, according to how clumsy I was at the time.
The boat slips over/through water lilies and weeds much easier too. Don't lay your boat in the bright sun bottom up. It will heat and soften the epoxy coating. It will reharden as it cools, but I would avoid that anyway. Scuffing the shiny surface with sandpaper exposes more graphite within the epoxy matrix; some think that makes it more slippery.
