Tumble drying nikwax does bugger all apart from re activate the original dwr coating. tumble drying the grangers restores the original performance. That’s why oxfam wash all their outdoor gear in grangers. Good as new!
The latest Grangers stuff doesn’t need to be heat activated, except for the two in one cleaner and reproofer. Nikwax doesn’t either, but their instructions say that heat treatment may improve performance, so if the care label allows, it might be worth it.
The latest TX-10 from Nikwax is claimed to last up to five times longer and it does seem pretty good, though it’ll still wear off in areas where the fabric gets rubbed, under straps and packs etc.
Tumble-drying, if the care label allows it, has two potential drawbacks:
1. If the thermostat on the drier is faulty, it can melt the jacket or bits like the zips plus kill reflective stuff.
2. Tumbling abrades the surface of the fabric, hence the fluff that gets caught in tumble drier filters. Maybe not such a good thing eh.