Most methods of removing rust are abrasive, and consequently also remove sound metal.
This can leave a smooth surface, but often this isn't important, and retention of the original metal is preferable.
A simple method of removing rust, is the electrolytic method.
It is ideal for heavily rusted items that are to be painted afterwards.
It turns the rust back to steel, rust scale is loosened and will fall off or can be easily removed.
Metal which is not rusted is not affected in any way.
(NOTE: In most cases, it will also remove any paint that is on the item.)
Stainless steel plate / bar
Plastic container large enough to enable the rusty item to be completely covered in the solution.
Add washing soda to the water at a rate of approximately 1 tablespoon per gallon of water.
Clean off an area of metal on the rusty item to enable a good electrical connection to be made.
Attach the NEGATIVE wire of the battery charger to the rusty item.
Attach the POSITIVE wire to the stainless steel plate or rod
Immerse both the rusty item and the stainless steel in the solution, and connect the battery charger.
If the charger has a meter, a small current will be seen to flow.
Small bubbles will rise from the rusty item, and within a few minutes the surface of the water will be covered with "froth".
The rust will be removed quickest on the surface facing the stainless steel, although all the rust will eventually be removed. However, it may be quicker to turn the item round so that both / all surfaces are facing the stainless steel.
The time required to de-rust an item will depend upon its size, the amount of rust, etc, and may range from a few hours to a day or so. If the rusty item is left in the solution longer than is required, it will not come to any harm, so the timing isn't critical.
In fact, neither the strength of the solution nor the current are critical, so it is a simple and "foolproof" method of de-rusting.
After the item has been in the solution for the requisite time, wash thoroughly in clean water and brush off any residual rust, paint, etc, that is still attached..
As the item will now be clean, and wet, it should be dried and painted as soon as possible, before it rusts again.