Stainless will rust when it is the secondary metal. Salt is a good primary and in a solution, stainless gets ate.
Get a blue 3M scotch bright pad and some mag wheel polish and work all of the spots out. If the spots are deep use sand paper: 1000 grit, 3000, grit, Red scotch bright, gray scotch bright, Blue scotch bright then a cloth with the mag polish. It will have a Mirror
I use the mag polish every month and it looks like a Mirror
! Everyone thinks I keep getting new mufflers! I also have a lift, so I don't need to get under it!