Cleaning the inside of a fuel tank is more time consuming than difficult but the big problem is because the tank is vented to a sometimes damp atmosphere, the more it is cleaned the quicker new rust will form unless it is treated or sealed. Below is a link to a UK supplier to provide an example of the sort of thing...
https://www.frost.co.uk/por15-car-bike-motorcyle-fuel-tank-repair-sealer-236ml.html
Begin with putting a pint of paraffin or kerosene in the tank and adding a pound of mixed hardware; nails, screws, washers, nuts and bolts and shaking for as long as it needs to remove the majority of the rust and dirt. Gravel chippings is an alternative.
Once you are satisfied the tank is fairly clean inside, empty, rinse a few times and then wash out with water and detergent. Dry thoroughly.
Remaining rust can be removed by using many different mild acids and the internet is full of ideas using substances as diverse as lemon juice, vinegar, tannic acid, phosphoric acid...
After soaking in the substance of your choice the tank will need washing and drying again before use or treating against future corrosion.
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