Stainless steel is a strong, robust and durable metal that far exceeds life spans of several other materials. Stainless steel is not just one material; it is a family of chromium containing steels, which all possesses an inherent resistance to corrosion. The chromium is distributed throughout the steel and its purpose is to form a sub-microscopic surface layer of hard, adherent chromium oxide on steel, a few atoms thick.
The film is known as the passive layer, and it protects the underlying material from attack by corrosive species. If the layer is damaged in any way, it instantly reforms in the presence of oxygen to protect the surface of the material." [http://www.caswellplating.com/bbs/] To polish use a mild polish (Happich Simichrome) to remove stubborn stains use an abrasive polish (Heavy Metal® Ultra Fine Abrasive Polish)
Exhaust Tips / Pipes:
To clean exhaust piping use a polishing powder (Blue Coral Blue Buster) to remove â€Ëœblueâ€â„¢ from exhaust pipes wet towel and polish, it will not scratch chrome, can also be used on brass and stainless steel.For exhaust pipe polishing use a strip impregnated with a metal polish (Luster Strips) (See also Chrome / Stainless Steel Exhaust)
Apply a polymer sealant for long lasting protection (Zoopseal or Klasse All-In-One)
I've had good luck with Mothers Metal Polish on my tips. Like Jon said, put a good sealer on after they are clean... it will make them easier to clean next time. Also, if you find your tips getting black quickly, your car may be running too rich.