A Challenger owner has as much chance of success as anyone.
How big is the affected area?
This may not be an easy fix by any spotting method depending on how large an area and what other contaminants might have been in the water. All you can do is try.
I would apply the Vacaway Rust Assassin is a small area, apply dry, (which it states on bottle, and I did not for the first few time, because didn't read it properly) and see if it comes out, if not, no use in trying the whole area, it is there to stay, if it separates than your good to use your challenger on it after the Rust Assassin, you use a pad and bone scraper for that.