i have had good results with painting the sinew bow back and belly with high quality outdoor acrylic paint and then poly over that,, protecting the sinew is good, but when you seal the whole bow it seems to help even more ,,I lived and hunted in texas and Tenn where it was very humid,, and the sinew bows were pretty stable when treated as mentioned,, I also refinished with poly probably once a year,, I have a feeling Pat is right on that the rawhide might attract moisture more than other backings,, snake skin might be a better option,,, or painting over the tight bond might work as well,,,