I was very interested in this subject years ago when I started hunting, and tried sprays, powder, and even using grey goose feathers with natural water repelency. After all is said and done, I would not discourage you from having the fun of working it out, but here was where I ended up:
1. If it is really raining, I stay home. I hate to dry out me leather stuff, and sitting in the rain is not pleasant while tracking is an issue of its own .
2. I found that the powders were just a PIA to use, but not bad at their job. It is just that the stuff is REALLY hydrophiic and gets on everything (because it is a POWDER) and annoying. The sprays work but seem to make the fletching more brittle and therefore noisier when rubbed up against.
3. I long since switched over to using a Cat Quiver, so except for the arrow in hand, the rest stay completely dry.