remember to de-grease first or you get patchy results. Store outside in child and pet proof box.
Citric acid is normally available as dishwasher and kettle descaler. You don’t need food grade and it is safe to handle and use in the kitchen or near children. It is sometimes suggested that you ‘damp it down’ by adding washing soda (calcium carbonate) to make a buffered solution. NOT caustic soda – drain cleaner- as it aggressively attacks skin.
For electrolysis mentioned above use washing soda NOT caustic soda.
Phosphoric acid is the best amateur cheap compromise. It can be used quite weak and apply by brush not dunking for best economy. After 5-20 minutes wash it off or leave for hours and it produces a crusty deposit and stains the workpiece but can be re-cleaned. It is available as swimming pool neutraliser and is hand safe in those formulations. Often the suppliers are resellers watering it down for more profit. My quick access jar actually keeps increasing in contents as it absorbs moisture. The dilution doesn’t seem to matter.
Sometimes Cocacola is suggested – this is because it used to contain phosphoric acid.
These agents are not a substitute for elbow grease – you must make the effort to remove all rust you can first.
After washing and drying on a good job you will see it quickly (2-5 minutes) darken as a fine new rust film forms. So as wuick a possible treat with well diluted clear Waxoyle car underbody treatment. This is much more user friendly than oil. Also soft candle wax (not car polish) can be used on hand and woodworking tools.