"No sinew backed bow is ever completely “waterproof”, granted it takes me a few hours to peel off sinew on a backed bow that has been damaged. But it does aid in keeping things dry. My belief is that they were clayed because of a bowyers spiritual prowess, hence the different paintings"
That is my thought too. George Grinnell wrote about the Cheyenne using gypsum over the sinew backing to whiten the bow. There is one of these bows on display in New York museum of natural history. A better water proofer I have used that I've also seed on an old Cheyenne bow and another Lakota bow in the same museum as well as some from the great basin is pitch. It also leaves a smooth surface and a nice black/amber color. If you mixed pitch with white mineral like the Northern California Indians did for a base that was painted over it would do a better job of keeping moisture out.