F9 should clean up the majority of those oxidation stains. But they will come back eventually. Something is causing them to exist. Looks like the lines from the solar heater are causing condensation or leaking. Stop or control that and you will resolve some of the future issues.
I would do a test before selling the job....or better yet.....as a way to sell the job. F9 does a great job on rust stains but I've not tried it on oxidation.
