Dodgy Fuel Tank And Not Giving In

Now the whole point of doing this rebuild is that I want to spend as little as possible doing it. Anyone can build anything they want if they throw enough money at it ( or throw enough money at someone else to build it for them !).

Ideally I would be able to do everything without spending any money at all but I know that for safety reasons or just common sense I will have to spend some money but I intended to use my own and hopefully your ingenuity and skills to reuse and repair the existing bike parts as much as possible.

This all sounded like a good idea at the time but one of the first things to come off the bike was the tank.
it looked ok from the outside, a little surface rust but not to bad.

Then I decided to open the fuel cap and look inside.

Two hours and much swearing later the fuel cap eventually came of, it had rusted itself shut and the inside of the tank is completely rusty and full of water.

Pretty nasty rust inside the tank
At least it does not leak

Now normally I would just scrap it and buy a new one but that's not the point of this project so I need to try and salvage this one and this is unfamiliar territory for me.
Cutting and reshaping, painting and stuff like that is no problem with the correct tools and equipment but rust well
I JUST DONT DO RUST.

I could cheat and buy a new one and just say it was fine all along but I am doing this as a challenge to myself and will be honest all the way through and anyway I do not give in that easily

So even this early in the project I am looking for ideas on how to de-rust and restore the inside of the fuel tank preferably for free or at least at very little cost and everything needs to be able to be done at home without specialist tools.

I will let you know what I decide to do and how it turns out in later posts but if any one has any ideas please let me know and I will chose one to try.