I've had packs where that stuff peels off the inside, too. To be honest, I dunno what it is. But the pack's performance isn't altered by it. The Nikwax will help the first burst of water bead up and perhaps run off. If it keeps raining, the pack cover and inside garbage bag will be your next line of defense. With those steps, your pack will be as waterproof as it ever was. Which, without the pack cover and garbage bag, is not very darned waterproof.
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