Pearl Drums... As for your questions it is obvious you have not worked much with sinew , rawhide, or hide glue. As many people believe a sinew backed bow can not be used in a wet condition, this is true to an extent, it can not be used in a constant wet condition, constant being the key word. Now sinew backing can not be "sealed" by itself. You can add rawhide or snakeskin or even canvas like some of the old Sioux bows had to cover the sinew and help seal out the moisture, which works to some extent. Sometimes clay was mixed with the hide glue to do this also. Hide glue can withstand alot of moisture before becoming damaged, you actually would have to soak a good sinew backed bow in water for a few days for the hide glue to give way enough to remove the sinew.
The rawhide on this bow does protect the sinew but was only added to give me a smooth surface to paint on, Now the paint being a modern acrylic will seal the rawhide as the paint is water proof. As for the water soluable hide glue it is there to put a layer over the paint to avoid the paint being scratched from an occasional scrape up against a tree or some other real world situation that we have all had while hunting or shooting.
If that does not answer your question fully feel free to p.m. me an we can discuss further the amazing properties of sinew, rawhide and water soluable hide glue.
Eric