Finally got round to rebuilding more calipers, so I thought I'd do the electrolysis thing.
All you need is a plastic bowl, a big sacrificial piece of metal, a battery charger, bicarbonate of soda and a rusty piece of metal.
Fill plastic bowl to cover rusty item. Put a tablespoon of Bicarbonate of soda in for every gallon water (about 5 litres).
Connect black wire (negative) to the item being de-rust-ified, and the red to the sacrificial metal (a big spanner in this case). You can submerge the black wire, but don't submerge the red wire, or it'll short and blow the charger:
Five minutes in and bubbles were rising from the caliper and the rust was 'flowing' to the spanner:
Half an hour later, and the water was turning a murky brown:
2 hours later, and the caliper was invisible due to rank water (I wasn't sitting there watching all this
I turned the caliper every three hours to get proper coverage (the side facing towards the spanner got cleaner):
This was about 9 hours of being in soak - there will be a very loose layer of rust which comes straight off with a wire brush or from scraping with a screwdriver:
Next time will use a proper surrounding metal thing.