when i picked up my used '11 F25 the dealership "cleaned" my car for me. it was the worst attemp i have ever seen for a delivery to a new owner..
i mention this because the soot was so bad on the chrome tips i thought their actual color was black. the following day i detailed the car as it should have been. when i saw the tips were infact chrome i broke out my can of Bar Keepers Friend.
http://www.barkeepersfriend.com/car-care
i used a moist terry cloth wax pad and made a paste, toothpaste consistency, and some elbow grease and they are as clean as the day they were installed on the car.
the comment about using never dull. i find that after a cleaning, never dull helps keep things cleaner because of the chemical it appies to the part, but it takes a long time to take off the carbon soot from the tips.
fjrmann