Waterproofing nylon jackets

the nikwax won't waterproof it, it improves water repellency making water more likely to run off the outside, but it can still be overcome by rain and water will still get through. it's debateable how much waterproofing you'll get from any treatment, you really need a tough membrane applied to the fabric at the time of manufacture to ensure water doesnt get through. tents dont have to be as waterproof as rainjackets, they dont get the same pressure applied to them from moving around, friction, wearing packs and sitting on the garment. I expect all the other treatments will do is boost water resistance and if they arent made for clothing they may wear off easily.

you dont have to get goretex, theres plenty of other options as good now. goretex is over priced.. all rainshells build up condensation, they are glorified wetsuits in the rain, breathability will never cope with a lot of sweat.

If its old its better to replace. There are plenty of fairly good pack away parkas for sale cheap these days. They are nowhere near gortex quality but will keep you dry and at most cost double the cost of your proofing spray. One that might work is the 3m silicon based fabric guard. but at $20+ a can is it worth even trying.

get a jacket with vented pockets or pit zips, they let the moisture out better than any breathable membrane