I have tried numerous way to water proof feathers. I happened to have some left over tent seam sealer and tried it on my feathers. It goes on as a thin liquid but sets up like silicone. The feathers are a bit more rigid but will not lay down. You can separate the individual strands in the feather and work the feather with your hand to soften it up. Top coat with some spray silicone and I think that is as good as you will get. The seam sealer I used is Coleman brand and is water based and will be odor free in a short time.
The feathers on the right arrow are treated and left are not.
