I've goofed up and damaged factory caliper zinc plating with iron-x before. But that was spraying directly on the caliper and rotors. Luckily it was my car and I eventually just painted them.
Ever since Ive stopped using iron x for wheels.
Definitely use it outside or smell it for quite sometime. I know you take care of your cars but you may be suprised at the results using it on your paint.
I think for the same reasons I explained up-thread. It works that well on paint. But as I said, I don't expect you'd need it on your own, well-maintained cars more than once or twice a year.