Snakewood, I think that the TBIII has better adhesion to the varnish than to the raw wood. At least that is my understanding. Shellac may also work, I don't know. Also, because Art said it works. He has never steered me wrong and has given me more good advise than almost anyone else I know. For me that is good enough.
Why exactly, you will have to ask Art. I don't have to completely understand something for it to work. If that was the case I would have miserably failed with wood bow building years ago.
Guys, lets just keep our fingers crossed about the success of this bow as well as the success at getting to GO elk hunting. Taking an elk is down on the list, going is top priority. Taking an elk on my second elk hunt is as predictable as winning the lottery...but I'll accept either with without reservation! d;^)