The most common way to remove rust stains is with an acid. Hydrofluoric acid is most effective but very dangerous to handle. Commercial rust removers most often contain oxalic acid. Bar Keepers Friend is a scouring powder with oxalic acid. Being an organic acid, oxalic acid is biodegradable and acceptable for sewer discharge. The abrasive solids might be an issue, but only if you were trying to use them on an industrial scale. Small use on an occassional rust stain should not be an issue. I don't know if any commercial products use citric acid, but it too should be capable of removing light rust stains. Once the stain is removed, you want to flush the surface with lots of water (to remove the acid) and if metal, apply a protective oil-based coating. Acid cleaned metal will quickly rust again if you don't take measures to protect it from air and moisture. Hope this helps.
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