Using a clay bar isn't the only option; there's also a much easier and more effective one. A microfibre detailing clay cloth and iron remover are all that's needed. While I like to utilise do-it-yourself detail supplies, you can save money by purchasing these options from Walmart:
Meguires - Towel for Detailing Clay with Microfibre Surface and Iron Remover
If the iron is embedded in the Clear Coat, washing the automobile will help remove it and prepare it for attacking the surface. Before lightly cleaning the car's surface, spray the stained area with waterless wash. Next, spray the rust spot with one spray of Iron Remover. Finally, spritz the clay cloth with one spray of Iron Remover. Just drag the towel over the surface without pressing down; the Iron Remover will do the rest. Scratches can be caused by rubbing or applying pressure with the towel. To remove brake dust and any other iron particles from a car, this approach is both easy and efficient. You may use this spot remover to the spaces behind the wheels and the wheels themselves when you've mastered eliminating this spot. The amount of brake dust and iron particles found on many automobiles' surfaces will astonish you. Every year when I detail my autos, I utilise this process, and the finish looks brand new again.