A call from Noel Mackintosh about cleaning castings dredged an almost forgotten memory about using a molasses solution to clean rust off steel parts.
It seems to use natural bacteria to do the job and does not affect any metal at all, even aluminium, but it is slow-taking about a week to do the job, depending on the thickness of the rust film.
The parts come out of the soaking bath so clean that they rust very quickly again in the atmosphere and need an almost immediate spraying with INOX, which I have found to be better than WD40, or undercoating.
At about this time, some 40 years ago, there was a Finnish product on the market which promised bacterial cleaning of oxides from all castings. I tried it and it worked very well being faster than molasses, but it very quickly disappeared from the market.
Molasses concentration is not critical and today I would use about one pound per gallon of water (1/2 Kg per 5litres) and grease should be washed off with solvent first.