Molasses is rich in chelates, chemical compounds which latch onto iron oxides and pull them into solution. Soak a rusty thing in molasses, the theory goes, and all the rust will go away. Eventually.
Molasses is also really cheap if you buy it from a farm supplies store. 12kg for about NZ$20. Mix it 1:3 with water, and soak away, stirring every couple of days to encourage the chelated rust to move away from the surface.
It works! It’s completely gross, but it works!
That’s what a tank of molasses looks like after sitting for 2 weeks. Not pleasant. It might have been less disgusting if I had left the lid off. The molasses won’t remove paint or oil, so I had to wash the block before soaking it. The engine paint was pretty crappy and came loose after a week’s soak, so a wire brush was enough to remove it.
Once the rusty thing has soaked for a couple of weeks, it comes out coated with black sludge, which hoses off easily to reveal lovely clean metal underneath.
Before and after shots:
I’m pretty happy. The results are better than the $100 hot tank clean we had done to our last engine, it cost $20, and the molasses bath is still perfectly useful for other things (I have the flywheel and pressure plate in there now).