I've successfully used a citric acid solution to remove rust. Light surface rust is removed in an hour or two depending on the strength of the solution, heavy rust takes hours and hours. Sonication speeds up the task, but is not necessary. Citric acid is a fairly mild acid that won't corrode most metals, is safe (use eye protection) and environmentally friendly. It won't damage most finishes, but I've not used it on a blued surface.

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