the old strap it to a wheel and go farming trick in one place, except I didn't here anything about putting a couple of hand fulls of small bolts and some sand in with a 1/2 gallon of diesel .
I wish my 8N was an NAA, the jubilee is a sweet tractor.
This sealer is commonly used and has been used for thirty or forty years (there are several brands).
Northern Fuel Tank Liner and Prep for all Gas Tanks http://www.jpcycles.com/product/900-715?utm_source=none&utm_term=[74945775370%3Aproduct_type_l1%3D%3Dvehicles%20%26%20parts%26%2Bproduct_type_l2%3D%3Dland%20vehicles%26%2Bproduct_type_l3%3D%3Dmot&adpos=1o5&creative=42616780330&device=c&matchtype=&network=g&gclid=COHpker118ACFVGTfgodr74ABQ
Had the same problem but used a different approach using the same method.
Filled the tank with a bucket of P-Gravel, then put the tank into an old oil drum and padded it to keep it centered as best I could. Then I paid my neighbors kid 10 bucks to push it around the yard with his go cart.
Well I use to remove rust by acid, muratic acid 3 parts water one acid . It desolves the rust down to the metal but not hurting the metal but then you have to neutralize it with baking soda . Now if there is a pin hole that is not weeping because the rust stopped it ,you will have one after so plan on brazing a hole or two.
The best, and most gentle way to remove rust is to use Phosphoric Acid..
I've done a good deal of engineering work in the metal coating business.. Whenever a paint or coating is to be applied to a freshly stamped or forged part, we almost always soak the part in a phosphoric bath. Usually its Zinc Phosphate but for those with heavy metal waste treatment concerns, we usually go with Iron Phosphate.
The phosphate turns microscopic iron oxides into iron phosphates... which are a microscopic crystalline structure that bonds at a molecular level.. That crystalline structure than provides a protective coating to the ferrous metal part and prevents additional oxidation.
Performing this same operation at home would be a simple task and I do it all the time.
Go to your local Home Depot type store and purchase any rust remover chemical.. It must say "Contains phosphoric acid" on it.
Use some soap and water to clean your fuel tank out.. rinse well, then pour in your phosphoric acid and water mixture.. Put the cap back on and be sure to roll the tank around to keep all surfaces wet for about 6 hours.
Stick the garden hose in, rinse it out twice and dry it the best you can.. Then immediately hit it with some isopropyl alcohol to help evaporate any remaining water that's hiding in any seams or gaps.
Let dry, and then do whatever you want. You could then coat it with any tank coating if you wish (I wouldn't), or just put it back on the machine and fill it with fuel.
This is how rust is handled professionally short of applications that can be sandblasted.
Gotta love science and chemistry!
Total cost for phosphoric acid will probably be around $10
I should add that if the rust is flaky and heavy, bouncing around some ball bearings in the tank to shake it loose would be a good idea. The phosphate would still work with heavy rust but you'd have to wait a long time and might require multiple treatments.
One nice thing about phosphoric acid is that you only need to neutralize it with water and if you leave it in there too long, it won't hurt the base metal that isn't rusted.
My father used vinegar and stones and by shaking the tank several times and leaving vinegar in tank for several days it removed the rust ,for better results after a good sand blasting will work also, needs a good rinsing and a good sealer after. http://www.por15.com/POR-15-Fuel-Tank-Sealer_p_64.html