We have some rust stains on the bow and stern and up in the anchor locker etc we would like to remove. So here is what we found out.
Do NOT use this on stainless steel.
We then found out about \uriatic Acid for removing the rust stains. Then we found out we actually had some on the boat! We were using it for removing the zinc from the plug that screws into the engine. That way we just bought a new zinc and did not need a new plug each time. Works great. Now we know it removes rust stains also.