Here is why the IWC steel cases you own, will propably last a very long while without being "stained".
Prolonged exposure in a saline tank....case samples are dissected and studied thru an electron microscope...for even the tiniest inclusions which would later show up as spots on the case. These will be rejected. Cases tested as a batch by batch basis.
Many companies do these tests outhouse, with contracters. But IWC does it all inhouse..for many years already, even at the old factory, where I took these pics.
The new premises has a cool homologation room.
More pics later!