Answer: A LOT! 3 x 5 litre full oil containers; 4 litres of water; and a tin full of 3/4 stones to be exact!
![Smiley :)](http://forums.sohc4.net/Smileys/default/smiley.gif)
Lesson?
If you're going to use electrolysis to de-rust your tank, do the outside first!!
![Undecided :-\](http://forums.sohc4.net/Smileys/default/undecided.gif)
Before and after internal pics attached for interest; it's not perfect, but ...
Sean