Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
Spirit of Salts or brick acid , I use SOS cleans it so well you need to add in caustic soda to neutralise the acid or it gets covered in surface rust instantly . Once the caustic is out then rinse with water , add in petrol/ oil mix. Best bit its quick , couple of hours tops and cheap £3 for 500ml is more than enough for one tank.
The tank has to be the sacrificial anode, otherwise you are plating the rubbish to the tank.
Herein lies the problem for us newbies who need to de-rust a tank for the first time - so much conflicting info everywhere...