Sander, I remove all the honey-comb looking winter growth that I can with a scraper lightly and then sand the back down really smooth. If you're planning to back the bow with something, then you may not want to sand as smoothly as you would if leaving the natural wood as the back. Especially with sinew where you might leave it rough for the glue to adhere to.
When we're working on a bow that is unsealed, during that timeframe of building it that may take several weeks, we leave it in a hot box or at a minimum over an airconditioner vent. All depends on how dry we think the wood is.
As for rawhide, you might get on the trading post section and see if anyone would be willing to trade you a piece.