Stainless steel supplied by reputable manufacturers, stockholders or fabricators will normally be clean and contamination free.
These items should not show rust staining, unless contamination is introduced.

The use of non-stainless steel processing and handling equipment is a frequent source of contamination. Work table bearers, lifting 'dogs' and chain marks have all been noted as causes. Non-metallic contact materials and vacuum lifting equipment should be used to avoid process contamination.

Handling or fabricating stainless steel on equipment, using tools also used for non-stainless steel should be avoided. Working in 'mixed-metal' fabrication shops, without taking segregation and cleaning precautions can result in contamination. Cutting or grinding debris from non-stainless steel should not be allowed to settle on stainless steel items.

As soon as any of this contamination becomes wet, rust staining will result.