Milltek say to use brasso, or silvo or something.
It does feel like it's made from quite thin sheet, and the silencers are made by two end plates welded around a T-joint to the sheet that forms the body of the can. All the welds look fine but you can see on the welds where the heat has discoloured the thin sheet. This may be completely usual... i haven't ever worked with stainless before.