Here's my take on rust:
It's best to prevent rust happening in the first place, but sometimes there's no way to do that. So if you have rust, there are two things you can do with it - get rid of it or convert it into something else.
Getting rid of rust can be done mechanically or chemically.
The mechanical method is as simple as it sounds. Knock it off with a grinder or sanding appliance. In the case of a fuel tank, you can't get at it with power tools so agitating it with abrasive material inside (such as pointy screws, nuts, etc) is about as good as you'll get.
To remove rust chemically, you just need a weak acid. Citric acid is good because it's non toxic and you can wash it down the drain when you're done with it. Phosphoric acid is just as good, or maybe better, at removing rust but I think it's more problematic to dispose of in a responsible way. Either acid will completely dissolve the rust. You can then hose it off leaving a clean metal surface. Generally washing it off is sufficient, but some people suggest using a neutralising agent like a weak caustic solution. I don't think that's necessary because then you have a caustic substance to get rid of. Perhaps a solution of baking soda would be harmless enough as a neutralising agent.
For the removal process to be effective, you need to completely fill the tank with the acid solution and let it soak for a couple of days.
Once you've removed the rust, you're back to a clean steel surface that may rust again in time. A little surface rust is no problem.
Converting rust is a chemical process. You can buy rust converter at hardware or paint shops. The basic ingredient in rust converter is tannic acid. It reacts with the rust and converts it from ferric oxide to ferric tannate, a hard bluish-black substance that's chemically stable. There may be other chemicals in rust converter to aid the process, but tannic acid is the main active ingredient.
With rust converter, you don't need to fill the tank. Just put enough in so that you can slosh it around to coat the entire inside surface. The chemical conversion process only takes about 20-30 minutes. It doesn't matter if you use too much. Just drain off the excess afterwards. It may need a couple of treatments if the rust is thick.
The blackish converted rust forms a barrier that offers some protection to the steel surface, inhibiting further rust.
The 'rust inhibitor' that comes in the fuel tank liner kit is probably rust remover, rust converter, or a mixture of both.
Preventing rust is some kind of treatment to stop rust forming in the first place. It's usually some kind of protective coating and can include paint systems, a tank liner, a coating of grease, spray-on stuff like fish oil or lanoline, electroplating, galvanising, etc. There's a whole bunch of techniques. Rust can also be inhibited by electrical means, such as a sacrificial anode. Sometimes products in hardware shops are mis-labelled as rust inhibitor when they're actually rust remover or converter. If you see a bottle in your hardware store labelled "Rust Inhibitor" either read the instructions to try to figure out what it does, or put it back on the shelf and choose a more clearly labelled product.
Back to fuel tanks ...
If you just want to eliminate fuel contamination from bits of rust, I'd suggest you start with a mechanical rust removal process to get rid of any loose flakes, then hit it with rust converter to change any remaining surface rust into ferric tannate. That'll give you a lining of sorts inside the tank. The black ferric tannate is fuel resistant and will protect the underlying steel from further rust.
