Just did the 20gal diesel tank on my tractor. First thing is wash it with a degreaser and rinse it out. Seal up all the openings except for the fuel fill. Pour in enough muriatic acid to slosh around and then go to town tipping it and rolling it around. Check it out through the filler every once in a while. Add more acid if you loose any. When it's de rusted to your liking neutralize the acid with a base (baking soda) and flush it clean with warm water. It will flash rust but it won't effect the red kote if you apply it. And if your not going to red kote it. You can give it a mist or fog of wd40 to keep it from flash rusting.
Gloves, cartridge type charcoal respirator and proper ppe are a must for this method. But it is the most effective way to get one clean if it cannot be dipped into a hot tank.
Steven