Got my 20gal/75 L tub full of water, got a nice clean steel rod (from an old stair runner, I think), mixed it up, hooked it up and works like a charm. Rust sloughed off an old battery tray like algae after an overnight session.
But here's the question: I bought a box of Arm&Hammer washing soda, which is 3 kg, or 6.6 lbs. According to Bob, he uses 1 lb/450 ml of soda to 4.5 litres (i.e. 1 gallon) of water, which in my tub would require 7.6 kg or 20 lbs of soda. Another online source
http://www.bhi.co.uk/hints/rust.htm says 1 tablespoon soda per gallon, which for 20 gals, would be about a cup and a half. I dumped in the whole 3 kg box, thinking it wouldn't be enough, but it seems like more than plenty- the reaction just fizzes away nicely.
Update: This with a 12V, 10-amp trickle charger, which is indicating 3-4 amps as it works.
So does anyone have the definitive proportions of soda to water? Or is it like martinis, mix to taste?