I did one application of the Nikwax Solarproof shortly after starting this thread, and want to follow up to say that it works absolutely excellent. I applied it with a spray bottle, and a small paintbrush, which I used to spread it around and apply extra coverage to seams.
I've done a bunch of long drives in heavy rain recently, and it still beads off (see photo). I really recommend this as a better alternative to the commonly used products because it doesn't have PFAS and it provides UV protection. I've also used it to waterproof a 20 year old tent that had lost it's waterproofing, which stayed totally dry through a rainstorm while camping.
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