No idea what it is, won't rub off, won't clean off. I have tried hot water, soap, rubbing alcohol, polish. Anyone seen this? HKS Super Turbo catback

 

Try some barkeepers friend. I use it to clean all of my stainless pots and pans.

 

Looks like acid-etched to me... Is it 'in' the metal (can feel the surface change with a finger nail)?


Could it be white paint? Driven on a freshly-painted street?


Try #00 steel wool next. Look up under the car too. Might be more lurking up there...

 

Looks like salt water residue to me. Does your state use salt on the roads?

 

Rubbing compound and then waxing will take care of that. I agree it looks like salt stain, or hard water mixed with something else.

 

Try some metal polish!

 

Flitz metal polish