If you remove any grease beforehand the molasses will work better. At least that has been my experience. I use a 6:1 ratio of water to molasses and if left for about 10 days it gives the excellent results achieved by Lynx42 above. A high pressure water-blaster (mine is 2000psi) is best for cleaning the black gunk off afterwards. However with my gearbox cogs there is still an unacceptable amount of residue that I now need to remove by some other method.
It is surprising how, sometimes the de-rusted steel does not start rusting again immediately after the molasses treatment, while other times it gets an orange "rust bloom" almost straight away. I suppose it might depend on the humidity of the air. I sometimes use a blow-torch afterwards to speed up the drying process and avoid further rusting. Its amazing to me that the molasses method is not more popular, as it does an excellent job, is very cheap, easy, biodegradable, non-toxic and (if you are in a rural setting) easy to dispose of afterwards. As far as I can see there is no down-side to molasses, as long as you plan ahead and are not in a hurry.
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