Beware with nitro finishes!
Bought on the recommendation of local luthier who did a setup on my new Martin D-18, which has a nitrocellulose finish, so I assumed he knew what he was talking about. I was wrong to assume that, though, as it only took a couple days in the soundhole of my D-18 to do damage to the finish all around the soundhole (haziness, etc.). It was not from water, either, as I meticulously ensured that the sponge was just damp (not saturated) and that the entire exterior of the humidifier was dry, even all of the crevices, before using it that one time. Rather, it was an interaction between the humidifier (vinyl and plastic) and the nitro finish. Luckily, with some careful but tedious polishing, etc., I was able to restore the finish aorund the soundhole. Needless to say, I will never again use this humidifier in this (or any other) guitar of mine, as it took very little time to damage to my beautiful new D-18. Shame on Kyser for not having a warning anywhere about the danger of using this humidifier, with nitro finishes -- guess I just had to learn the hard way.