ive been doing some research and smoking the rawhide may give it some resistance to moisture, much like it does to leather in the brain tanning process. after that its pretty much up in the air, some say hide glue some say shellac or varnish to waterproof rawhide. We know hide glue isnt waterproof, so thats probably out of the question. shellac or varnish im not so sure of, it may make the string very stiff. wax would probably work well if you use a piece of leather to rub it in, getting some friction heat to make the wax melt into the pores.