My stacks on my tractor trailer had rust stains on them where condensation from the exhaust ran down the outside. I polished them yesterday with Autosol polish. Simply rub the product on the let it sit for a minute and then buff it off. Most rust should come off first try but if it doesn't just try another application. Eventually it will all come off and stay off if you give it an a cleaning every other week with it.
Autosol® Polish in a Tube Item # ASOL1 - Winter Brook Farm Antiques
One tube will do your exhaust tip for a couple of years.