Thats what i,m sure i read on DW about this although of course you dont wanna be rubbing dirt around with clay . These iron deposits were trying to remove can sometimes etch themselves very slightly under the actual surface so no claybar will get them. Just be sensible about it, clay after to make sure everything is definately gone yes and if its caked in bad iron contamination then use IX first after your wash process . You dont want to clay a load of 'large' filling about on paint
After claying a bad suface it may well be in need of machine polishing to correct the marks .