Hi Blumbly,
I have used Apple Cider Vinegar to clean out a rusty fuel tank and it works. Buy it from Coles or Woolies, soak for a day and empty it out and have a look. You can judge how long to leave it in for. The fuel tank on my stationary engine was well rusted and it did a great job. After the vinegar treatment you can use the gravel, it's good for getting rid of some loose bits.
I had the same drama and Googled looking for help and a lot of people have had success with the vinegar, normal vinegar works but not as well. Good luck.
