De-rusting with electrolysis: the details

This is such an interesting subject I am putting as many pics and details up as possible. I got the idea from Mike Brown's excellent 2009 book Building Budget Brits, which is essential reading for anyone fool enough to own a Triumph or BSA unit twin.

Here's the method.

1. Shake nuts, bolts or gravel around in the tank to dislodge any big flakes of rust. Use a cement mixer if you have one. Remove with old vacuum cleaner.
2. Make up solution of tablespoon of washing soda to a gallon of water.
3. Bung fuel filler holes with Blu Tac and fill tank to brim.
4. Hook up a battery charger, negative lead to the tank body, positive lead to an iron anode suspended over the filler hole, in the soda solution but not touching the tank.
5. Some chargers have cutouts that won't let them pass current when used like this. I used an old mobile charger delivering 500mA max.
6. Leave it fizzing until the reaction appears to stop. It was a few days for me (plus a couple of weeks before I had time to work on it again), but heavier current might speed it up a bit. The rust is being converted to an inert blackish substance. I don't know what the chemical reaction is but I guess it's reducing (de-oxidising) the rust.
7. The process deals with even the remotest corners of the tank. But once you turn off the current you must dry it asap. Mike recommends pouring out the solution (which is not particularly toxic – I dumped it on the gravel drive) and flushing with acetone to chase out the water. I used cellulose thinners (similar thing), then sat the tank in front of a fan heater for half an hour. Dry as a bone.

Did it work? See the before and after pic and judge for yourself. I think so. The orange, rusty smudge you can just see is only under the electrode, where very fine particles of brown sludge fall downwards. Otherwise the entire inside is grey or black.

Obviously it's less stress if you do the job before painting the outside, but the solution (which fizzes over sometimes when a big hydrogen bubble forms inside) is non corrosive on paint and chrome. Also, it helps to scoop out the scum and clean up the electrode (see pic) from time to time.

You can now seal the tank but I'm taking my chances as it is.