Lots of ways to skin a cat -- I like the washing soda electrolytic method having just tried it. I've also seen sodium bicarbonate (baking soda) used, though it's more expensive, it's even more widely available and still cheap enough.
What I used, was sodium carbonate -- available everywhere at supermarkets --used for clothes washing, and the cost was 15 cents for this tub full of solution -- which can be re-used. In fact I have a big old dividing head that needs to be restored, and that will be next in the tub, plus a bunch of other rusted odds and ends. But chemical methods like citric acid seem good, too, if you have it. Whatever works!
re. explosive gasses and safety -- Charging an auto battery (with the same charger) also liberates oxygen and hydrogen, and in this case I was drawing less than an amp, so the gas evolution (charge) rate was low compared to an open lead acid cell.
I kept the shed door open, and the tub in the open doorway itself, so essentially it was outdoors. (the shed used is unfortunately "ventilated" in the extreme, even when closed!) I also didn't leave it unattended or over night. I checked it periodically, had it on a 2 hour timer on the charger, and that's how long it took to clean up. I always turn off a charger before removing battery clamps from a battery -- learned that one when I was 19 the hard way -- and I did the same for this.
Just including the above in case someone else decides to try it as well.