The short answer is: no, you can't.
Waterproof jackets use a membrane (or PU coating) that makes them waterproof. We use GoreTex and Pertex membranes in our range. To be fully waterproof the seams also need to be taped.
Spraying your down jacket with a waterproofing agent such as Nikwax TX Direct will only apply a layer of DWR (Durable Water Repellent) coating to the fabric. (Just in case you didn't know, there's already one on there!)
Why don't you make your down jackets waterproof?
There are some great reasons for that.
Firstly, down jackets are made for cold, dry conditions. Here it tends to snow rather than rain. The DWR coating is enough to shed any snow that lands on your jacket.
Secondly, waterproof fabrics are actually pretty heavy. For mountaineers, weight is an important consideration. The lighter, the better. Adding a waterproof membrane on our down jackets would add unneeded weight that wouldn't add to the performance.
Do you have any waterproof down jackets?
Yes! We do.