Living in a very humid part of the country, St. Louis, I have to say that the humidity in and of itself it going to be your problem. The houses here, no matter what the siding material, get mold/mildew build-ups. You can't be getting all of the moisture out of your garage unless you basically dry the walls, ceiling, and every surface in the garage each time you use your tub. If the siding on our houses grows mildew, mold, etc., outside, then you are going to have problems. Just because you barely feel moisture or however you phrased it, you aren't touching all surfaces and over time this is going to create problems. I would think that the moisture will most likely cause problems with some items in your garage that rust and coupled with the chemicals it might make things worse. But, you will most likely have real mold problems at some point in the near future on all of the garage surfaces. Even with excellent fans venting moisture out of bathrooms, over time you get mold and mildew build up.
