You can try cleaning with very fine wet and dry and follow up with stainless polish and stainless cleaner to maintain. A bit labour intensive, but you will have the satisfaction of a job well done. :clap: I cheated on my OEM muffler and used 6" buffing wheels on my bench grinder, for the hard to get places I used a air grinder with cotton mop attachments. You have to be careful with some aftermarket mufflers they are not built as substantial as OEM in wall thickness, and can ding easily. I do seem to remember somewhere BMW recommending OEM mufflers be cleaned with French chalk only.
Regards Martin.