When we detail the tips, the best arsenal for us has been a multi-stage process. After washing, we spray the tips with P21S Wheel Cleaner and begin cleaning with OOOO Steel Wool. It sounds a bit harsh, but on polished tips, it works very well. Once you get all the discoloration and carbon deposit off, we then polish using Zymol Metall. It's an excellent procedure that works like a charm. You have to be more careful on chrome since you don't want to used the wool on that. For chrome, we clay the pipes and use Zymol Metall to polish. If the chromse is really bad, you can polish it with a product like Venol or Simichrome. Oh, unlike Blue Magic which is a very harmful product to use, both for yourself and the environment, the products used, P21S Wheel Cleaner and Zymol Metall, are enviro-safe and safe for you.

 

I've used Wenol metal polish on my BMW, and it works wonders on dirty exhaust tips. Finish with a coat of sealant like Menzerna FMJ and it will make future cleaning easier.
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Using sealants such as FMJ will seal the pores of bare metals such as brushed stainless steel or aluminum. This will adversely tarnish the metal since the metal cannot breathe. As long as you do a good initial detail, a simple spray of wheel cleaner such as P21S Wheel Cleaner and gentle agitation with a soft wheel brush should do the trick on subsequent washes.