+1 to that. Ultrasonic and chemical cleaning will remove a heck of a lot of "stuff", but there are times when a bit of manual intervention is necessary. I have a brass, steel and fiberglass scratch pen set (cheap 'n Chinese, from ebay, as is everything these days it seems). The brass pen works well on stainless cases, as it is softer than the steel, so doesn't scratch the finish, but will remove even the most stubborn bits of ancient DNA and fossilized chicken soup.