So it seems like citric acid, rinse, then phosphoric acid. I can get both on the net here (phosphoric from home brew suppliers!)
Not sure I´d try electrolysis. Was that a steel nail or a gal one?
There's no deep rust in this tank, but powdery surface rust. I tried rust converter after a net search "best results ever!", was a stinking mess from it, now dried out and gone and the powdery rust is still there.