Another good solution, i just did mine a couple of weeks ago
Get about 4 or 5 litres of Apple cider vinegar, (normal cider vinegar works also i assume)
Swish it around n the tank every few minutes then after about half an hour empty it out and do it again if needed.
mine was pretty bad, so much that i cut a access hole so i could give it a good scrub, but i found out about the cider vinegar too late,
it literally rinses the rust away i couldnt believe it.!
I painted tank bay and tank with Rustoleum Rubber spray, after a zinc primer, its good cos it dampens vibration in the tank as well as keeping rust away
