I have this friend. And he was building a piece of furniture, and messed up with his Domino cutter. So he asked me if I knew anything about how to plug a Domino mortise so it wouldn't be so noticeable. I told him I had no idea since I had never made a mistake like that, but would ask here. He needs to fill some edge grain red oak, and it needs to be cosmetic only, since he can switch the board around and cut into fresh wood at the proper spacing.

