Overview of passivation:
http://www.pfonline.com/articles/119806.html

The pertinent sentence from the above page:
"In lay terms, the passivation process removes "free iron" contamination
left behind on the surface of the stainless steel from machining and
fabricating."

Nitric acid process for passivation:
http://www.corrosion-doctors.org/Mat...assivation.htm

Citric acid process for passivation:
http://www.pfonline.com/articles/059905.html
The citric acid procedure sounds doable for a homeowner if you can get
the acid to stay in contact with the SS long enough. Maybe a cloth
saturated with the citric acid solution can be laid on the SS surface
for 20 minutes or so.