As SuperCheap Auto were having an Easter sale today, I popped in and got some supplies for doing up my tank, including wire brush that fit onto the end of my drill. It worked so well to get rid of any surface grub and rust on the tank before I put on some reinforced bog. I am waiting for the bog to cure before sanding now. This putty as stainless steel fibres to reinforce it.
My tank all nice and shiny after the wire bush treatment. No more rust :-) At least for now!