So my 24-70 II did the same thing over the weekend. We has a slight mist while shooting Football. The nano coating came off when I wipe the excess moisture off. Canon is saying it is water damage because I was honest and stated it happened in a mist. This is very disappointing. The lens is advertised as "Highly dust and moisture resistant." I have been hung-up on, disconnected and chat and given the run around by Canon Repair center employees. I am not happy since canon wants 432 to replace what should have been covered under warranty.
But it is highly moisture resistant. As in, you'd have to be high to think it was moisture resistant. That's what they meant, right?
But seriously, any lens coating that can't tolerate water also cannot be safely cleaned
, which by definition makes it a fundamentally defective lens coating whose very existence does far more harm than good. If Canon refuses to cover such an obvious manufacturing flaw under warranty, it is highly likely that Canon is breaking the law.
I'd like you to try contacting them again, this time by certified mail with a return receipt. Point them to this forum thread, where a number of people have reported having the same failure with that specific lens. When you do, be sure to mention the magic words "Magnuson Moss", "fraud", and "class action". I think you may have better luck with that approach.
If that doesn't convince them to honor their warranty, I would strongly encourage you to contact a lawyer. Ask for punitive damages, and treble damages. After all, if they've been presented with evidence that this is a fairly common failure that is not caused by customer abuse, and they still refuse to cover it, that constitutes a willful violation
of the law, which is much, much worse than just breaking the law.
Caveat emptor: IANAL.