Wouldn't the wash-in Nikwax waterproof the inside too thus making it less breathable?
Why isn't spray-on as popular?
Is spray-on sometimes better than wash-in and sometimes worse?
Overall what do you recommend?
The Gore-Tex site recommends washing the garment with regular laundry detergent in warm water and then drying it in the dryer at a warm temperature, basically doing the same thing as you would with any clothing. The heat of the drying process reactivates the DWR. I've tried it and it works for me. My jacket is several years old (maybe 10?) and I wash it a couple of times per season. It still works like new.
I do what Posaune does, just normal washing of Goretex but then I spray "Kiwi" performance fabric protector on the fabric for increased water beading. According to the label: "Restores Durable Water Repellency (DWR) on apparel to prevent "wetting out"."
Between washings on cuffs and storm flaps etc. I use a spray on fabric cleaner and rub with a clothe from that British company Granger (?)