I have a PS PRS acoustic with a nitro finish, but have also had PRS, Martins, and Collings with nitro in the past.
My whole-house humidifier is unable to provide sufficient humidification for the guitars. However I also humidify the room I keep the guitars in with an Air-O-Swiss s450 steam humidifier. I keep the R/H around 40. It's enough to prevent problems from lack of humidity.
Probably like you, I've experimented quite a bit in keeping guitars humidified because I'm alarmingly OCD about the guitars. Here's what I've found works for me, YMMV:
1. Case and guitar humidifiers haven't done me well over the years. I've just never had acceptable results using them. I've had the long green tubes you wet and stick in the guitars, the ones that clip onto the strings, the blocks of plaster in a little case you soak to absorb water and stick in the guitar case, and in the late 60s even followed the traditional advice of sticking a potato in a plastic bag pierced with holes and then put in the case, which is the worst idea anyone ever came up with, took months to get rid of the potato smell...anyway, none of these worked well for me, and I found that if you put enough moisture in the green tubes you stick in the guitar to matter, they drip inside the guitar causing a different problem.
2. I've found that a good room humidifier works, but getting the right one is essential. If the room is at a good relative humidity (R/H) level, it stands to reason that the guitar won't know whether it's spring, summer or fall, when you wouldn't put a humidifier in a case anyway, at least in my part of the country (Michigan). Michigan is humid in the summer, decent in spring and fall, not too dry, but a house with forced air heating gets worse than the Sahara Desert in winter. It gets really cold here in winter. The Sahara is 29% R/H. My house has gotten down to 23% R/H in winter, even with a whole-house humidifier!
My findings re: room humidifiers over the years:
- The ones with the rotating drums start smelling after a few days, have the dreaded white dust, and are a real pain to fill. I've even had the expensive German rotating disc ones that didn't really provide much humidification and were also noisy as hell. You're dealing with bottles of liquids to add to the water to prevent mold and mildew, difficult cleanups of water buckets that get greasy with slime, etc. No thanks.
- The electronic mist humidifiers do inevitably cause white dust (even the ones that aren't supposed to, such as the Air-O-Swiss) and worse, the white dust tends to be attracted to one's electronic equipment as the particles of dust appear to be electrically charged and seem very much likely to stick on audio or video gear, in the crevices and vents of the equipment mostly. So, yes on the easy to clean and maintain, no on the white dust particles that are attracted to your computers, speakers, and other electronics.
- I've found steam humidifiers to be very reliable and work well. I've tried several brands of these. The good ones have an absorptive pad to prevent the white dust, and I've found them to be
far more effective at preventing dust than other humidifiers.
They also work better at providing humidification. With the Air-O-Swiss I'm able to set an R/H and can check it with the built in hygrometer, though I have a second hygrometer just to make sure because,, yes, I'm that OCD. They're easy to fill, the downside being that they're a little harder to keep clean than the misting ones because the evaporated minerals tend to build up on the boiler surface and the chimney where the steam comes out. The Air-O-Swiss is made of a fairly slippery plastic inside, and I find that it's the easiest of the bunch to clean. But in any case, these are far easier and less disgusting to clean that the drum type, or even the rotating disc type because I'd rather wipe off a few crumbs of dried-on minerals than deal with gooey slime. Just a personal preference.
Every few days you have to refill the water tanks. Big deal.
- Regardless of the type one uses, the downside of a room humidifier is that if you set the humidity too high, you can cause ordinary household mold to appear in corners and around drafty windows. I've found that as long as I keep the R/H around 40 and no higher, this is NOT a problem, and is enough to prevent damage to the guitars. I have also started using a "peel off in spring" caulk around the edges of the windows to prevent drafts. This means more heat is retained in the house, which reduces the problem of the furnace going constantly, thereby helping with humidity.
