Prep1957
I just posted this on the Sovereign forum for Gunnar.
The formula works well and can be made up in smaller quantities, just keep the water molasses ratio about 10:1
Get an old plastic trash can and fill it with about 10 gallons of water.
Go down to the local animal feed store and get a couple of quarts of horse feed molasses
Mix the molasses into the water and add a bit of yeast, or a can of beer, or a couple of slices of bread. Any of those will do, but the yeast is quickest.
Put the lid onto the trash can and forget about it for a week or so, by which time it will be fermenting well with a scum developing on top.
Now all you have to do is immerse your iron or steel object in the solution. Make sure it is completely under the liquid.
The process is gentle and somewhat slow, but after a week or two, depending on the amount of corrosion, pull it out and hose it off and give it a good scrub with a wire brush or similar.
The black deposit will come off fairly easy, if not put it back into the brew for another week or so.
Do the brushing off outside wearing old clothes as the black residue will stain and the stain is permanent.
Interestingly the black deposit is almost identical to the printers ink used for printing in the early 1900s.
When done properly the iron/steel object will be perfectly clean, it will look just like it has been sand blasted.
The object will in fact be so clean that you can see the oxygen in the air attacking the surface. You will see the surface becoming orange with a film of rust before your eyes.
To prevent further rusting or oxidization, brush a coat of rust converter or phosphoric acid solution over the surface and it will not rust again if kept under cover.
This process is used by people restoring old cars and motorcycles in Australia and I guess other parts of the world. It is particularly handy for de-rusting small difficult to get at things like inside petrol and oil tanks.
It is important to fully immerse objects as you will get severe pitting at the air/liquid interface otherwise.
The brew lasts for months if not years.
Best of luck
Pete downunder