[Bob's GTO]

Looking for suggestions or ideas. Last week I got caught in the rain coming home from a show. Before I pulled into the garage, I rinsed everything underneath the car with water from my garage hose bib. A couple days later I noticed the gas tank had water spots. This is a new tank from Yogi's that came Powdercoated a nice silver gray. I tried some compound in an inconspicuous spot, it took the spots off and some of the finish. I then tried Lime Away which did nothing. Does anyone have any suggestions on how to clean the waterspots off the powdercoat finish?
OBTW, yes I have well water that tested hard, I have a softener system.