A friend of mine told me about using MOLASSES to remove rust stains on stainless. Yes you heard right MOLASSES!.
I gave it a go and it worked leaving the polished stainless unmarked. I painted it on left it a couple of days before hosing it off - perfect.
It appears that the reducing sugars in the molasses causes the iron III to be reduced to iron II (soluble).
Thought this might be useful
Regards
Aquapore