Citric acid can be bought from drugstore (they use it in jelly or such to adjust acidity) but it is very expensive there. Therefore Finnish rodders buy it from some agriculture-store. It is not expensive. Officially it is used as pigs obstruent! I dipped those parts in citric acid for overnight, next day brushed it with wirebrush and flushed it with water. Then again back to citric acid over night and wirebrushed + flushed it next day.
Flushing is important, you’ll want to get all acid away, otherwise steel will turn to yellow.
You can’t use this method on all parts because of hydrogen embrittlement. So no pedals, frames, brake drums, engine blocks or such to citric acid!!! And yeah, parts should be clean -I’ve cleaned ‘em with fuel- because grease will neutralize acid. Ratio was 3%
Parts must be dried soon as you get them flushed, I quess you guys know why…