This is a seriously old thread, but the acid approach was the right solution. Oxyclean removed most of it but not all of it. Sunlight took care of most of the rest. What finally got the last of it was a phosphoric acid wash. Our andchor chain rusted badly last year in the
bahamas as the galvanizing gave up after 10 years. The result was a badly stained foredeck (rust stained). We were looking for solutions to that problem and found that phosphoric acid took it off and left a nice white
deck. Unfortunately I had only a small amount available at the time and could not treat the whole area. when we got back to the states I bought some Krud Kutter phosphoric acid at Lowes. I diluted about 3 to 1 and sprayed it on with a garden sprayer. I let it sit for 30 minutes, rinsed it off and the entire foredeck was lily white. It was only then that we realized that we still had some stain left from the
boat yard incident as the unsprayed area still had a slightly reddish hue. When there was no pure white next to it the stain was not that noticable. So I bought some more and sprayed the entire
boat. Now it's so white it hurts my eyes on a sunny day. I imagine that lemon juice or muriatic acid might have been just as effective, but I was a bit reluctant to use those as I have seen what HCL does to metal and to be honest hadn't even thoght of the lemon juice. Phosphoric acid is commonly used to remove rust so I wasn't too concerned about the metal and any rust
removal from my boat is a good thing. It even took the rust stains off of the side of my boat from the not so
stainless steel ports. My boat might even be allowed out of
New Zealand now.