what is the best way to seal up and waterproof a bow backed with sinew from a natural NON waterproof hide glue?
I put snakeskins over the back of my one and only sinew backed bow. Sealed it with Tru-Oil, think I got the idea from Jay Masseys The Bowyers Craft book.
You can not waterproof any wood bow, sinewed or not. You can slow the hygroscopic rate with different finishes. I have also used snake skin with Tru-Oil with good results. Jay Massey started using French polish(linseed oil and shellac) on all his wood bows, sinewed and selfbows. He hunted Alaska so they had to repel water well. Later he used epoxy cut with a solvent(Massey finish).
If you know you are going to be in incliment weather wax your bow well with paste wax or animal fat to repel water. No matter what you do or how you seal your sinew backed bow humidity will eventually get in and soften the bow up. A few days in dry heat will release that moisture out of the bow.
Tru Oil has worked great for me in self bows. Paraffin works for too
although I have not tried it. Tru Oil is great on sinew and snake.