With Caustic Soda
This process will remove rust, grease and paint from steel or cast iron parts. The part is left in the bath for 24 hours and on removal from the bath is hosed gently whilst being rubbed with wet and dry paper or a wire brush. The process is extremely effective if the points below are closely followed.
Any type of container is suitable provided it will remain water·tight during use, Steel containers should have a plastic sheet inserted before filling plastic containers must be strong (i.e, plastic rubbish bins tend to split). Obviously, the container size limits the size of the part to be cleaned; large parts may be cleaned one half at a time.
Carefully and slowly add caustic soda flakes into warm water and stir until dissolved. You will need approximately two kilograms of caustic soda for 20 galls (91.2 litres) of water. (A 2.5kg tin of caustic soda flakes may be purchased from any hardware shop). The caustic soda flakes, may be dissolved in cold water if you wish but allow 24 hours for your bath to start working effectively.
The part to be cleaned is connected to battery charger negative, Battery charger positive is connected to a piece of steel, say 3" x 3", You will find that a 3" nail is suitable for the positive electrode when the part to be cleaned is large (i.e. bonnet), otherwise the current will be too high for your battery charger (general purpose charger delivers 4 ampere at 12 volts). For both electrodes, a good electrical connection is required (THIS IS IMPORTANT).
At least one of the electrodes should be supported from a piece of wood so that electrodes do not touch.
The part may be left in the bath for days with no damage but 24 hours is long enough for most parts.
Under no circumstances allow the caustic soda solution to touch your skin, I suggest that you purchase a cheap pair of waterproof gloves. The bath should preferably be well ventilated and definitely inaccessible to children
Morris Register of Victoria Newsletter, February 1994, Vol 17. No. 5
Everyone has heard how molasses dissolves rust, so make a trip to the local pet food and grain store and get your molasses.
The formula is two litres of molasses in 7 litres of water. Put this mixture in a plastic bucket or container and partly cover to help stop evaporation. Leave for about three weeks, down by the back fence (it pongs a bit), until it ferments. It should now have a skin on the top, which should be peeled off. Now you can immerse your rusty parts in this solution. Leave for about two weeks before removing them, by then all the rust should be dissolved (use rubber gloves, long tongs, or tie pieces of wire to the parts before you start, as this mixture contains ACETIC ACID).
After removal, wash off the brown muck straight away with a stiff brush under hot running water. As soon as the parts are dry, treat them with rust converter and paint them as soon as possible, or if not painted, wire brush and oil them. This must be done immediately because surface rust will start to form as soon as the metal is dry, because it is so clean it has no protection.
Apparently the water and molasses mixture when left exposed to air, ferments and produces, amongst other things, Acetic Acid. This reacts with the oxygen in the rust and when the iron oxide (rust) is all reduced the process stops, so the steel or iron is not affected, but the surface of the metal is now virtually in original condition and subject to immediate attack by oxygen in the air and begins to rust, so must be protected.
The benefit of using molasses is that it dissolves that rock-hard rust that even wire brushes can't touch and carborundum cloth can't reach and by using arrangements of odd-shaped containers like old concrete troughs half full of dirt and lined with heavy plastic sheet, it is possible to derust larger objects that would not stand sand blasting.
This mixture will still derust for quite some time, (six months or even more).
"Talking Shop" - Crank Talk, September 1991