The other day I was trying to turn a demo piece for a router class I'm doing and the wood apparently had a weak spot and blew out 2 small sections as I hollowed it. Not wanting to throw it out and needing to practice this technique again I patched it. As you can see in photo 1 it wasn't 2 awfully bad an not terribly thin. I inflated a ballon inside the vessel to act as a dam to keep the epoxy from running all over the inside. Then I mixed some epoxy and added thickener. I also added some paint to roughly match the color I wanted. I coated the surface and let it dry. See photo 2. In some cases what I've done is to drill out a small hole near the middle and add some more epoxy of a different color to sort of simulate a knot. I've also taken a thin permanent marker and made a small line hear or there to look more like the colors in knots. I have also been known to take the Dremel and reshape the torn areas. In this case I didn't do any of those. Here is a photo of the final piece repaired and the final piece. Notice there already was a real knot in the piece.