I've tried potato, dental base wax, and PlayDoh for plugging tubes before I epoxy them, but have found that non-hardening modeling clay from an art supply store works best for me. While it does keep epoxy out of the inside of the tubes, it can still build up around the outside and needs to be cleaned off. I use drill bits for this, and a small scraper if necessary (which is actually the awl from a pocket knife). I made up a chart of cleaning drills of the closest diameter for the inside of the various tubes I use. As you can see, some are closer than others.
I don't know why the photos uploaded out of order. It's happened to me here before.
