Once you're satisfied you've cured your tank of its ills, it might be worth fitting new seals around the filler and the cap. The problem with flush fitting fillers is that rain water will inevitably find its way past the seals, if perished, even more will get in and the water ain't going anywhere except to the bottom of the tank where it will sit and rust your tank A friend used to dilute hydrochloric acid - tipped out from old batteries - and a quick swill (not long enough to do any harm to anything except rust) would almost instantly restore a tank to as new condition
