Lex, it's from the rain. Mostly on the back glass. I never have a problem on my paint because I keep such a good coating of wax.
Denny, I'll let you know how this works. Everything I have ever gotten from Zaino was awesome.
Once I took my S2000 to the casino and parked it overnight. I parked way out like I always do, but didn't think about the sprinkler system coming on and hitting it. They must just pump that water straight out of the Mississippi river because it etched spots on that car that NOTHING would remove. I tried everything. Clay bar, scratch remover, abrasive wax......nothing removed it. I was sick. Of course, only I could see it.