Wayne Maxam says to tell you there is a product in the paint dept. at Canadian Tire called 'Metal Saver'. He says he has been using this with good success on rusted tanks. Four litres costs $25 or so, and they have been filling tanks with the solution, so depending on the tank, more than one might be required, though it is re-usable. I thought it sounded like phosphoric acid, but Bulldog says it leaves a shiny surface, and phosphoric acid leaves a whitish haze, so I don't know what this might be.
The tank lining kits come with phosphoric acid to etch the steel and dissolve existing rust, and it is not hazardous in any serious way, which Wayne also says of this 'Metal Saver', and contrasts with some of the more adventurous acids, like muriatic - which I have always shied away from. The electrolytic removal process sounds good, but access in a motocycle tank through the filler can be bad...
Addendum: Wayne left a message saying the stuff he was recommending is, in fact, called 'Metal Rescue' rather than Metal Saver.
