If you go to the camping section at WalMart they sell stuff called Silicone Water Guard.

It's an aerosol water proofing solution for camping gear.

Spray some into a cup to concentrate it. Dip flies in it, and let it dry overnight.

Waterproofs everything. Lasts longer than I've been able to fish the same fly. Especially effective on deer hair bass bugs.

I tested a lot of things, and this is by far the best I've found.

Everything I've read about WD-40 and fishing has been unanimous that t's not good for the water, doesn't work as a 'floatant' and contains no 'natural' ingredients like banana oil (what it smells like). No one in the bass world uses it as an 'attractant' anymore, not because it 'bad' for the environment, but because it doesn't work near as well as the stuff made for the purpose.

Buddy