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Just seeing what the STW masses use on their down jackets to keep up the 'DWR' finish after a wash or after a period of time etc. I have a Patagonia Down Sweater? Cheers
I'd be using the spray-on version of Nikwax TX-Direct for the shell fabric. Just follow the instructions, but don't expect miracles, down's not ideal for wet weather use.
If you wanted to make the down itself water resistant, they also do a wash-in down proofing product, which is the domestic version of the treatment used by some outdoor brands including Rab, I think, but the whole process of washing and drying down is a little fiddly, Basically you need to agitate the down while it's tumble-drying at a low temperature to stop it clumping. A couple of tennis balls or similar should do the job.