Treating a down ZPacks Bag with a waterproofing agent

[ndbuck09]

Has anyone ever treated the outside of a down bag with a waterproofing agent to give the outer material resistance to water/moisture? If so what have you used? how much did it increase the weight? I have a couple years old ZPacks 20 degree bag that I'd like to treat.

Thanks

 

[ndbuck09]

given some threads about down and dryness, I wanted to bump this up to see if anyone has tried treating their down bag in some sort of waterproofer?