"Iron" Stain on New Cedar Deck

Late last fall, we had a new cedar deck constructed. We designed the deck with an iron spiral staircase. The day after the staircase was installed, I noticed massive "black" spots on the cedar boards. (There was some grinding/filing that needed to be done when the staircase was attached.) I could even see boot prints on the boards. Several people have told me those are "iron" stains from when the iron came into contact with the "wet" cedar. I really need some advice as to how (and what product to use) to remove these stains.