I’ve never used ammonia but bought some to try. My rusty tank formula is:
1. Acetone left in a couple of days and sloshed around to cut the hardened hydrocarbons. Repeat as needed.
2. Phosphoric acid rust treatment ("krud cutter," "phosphos," etc) shaken up with stones, until inside of tank is shiny.
3. Hot water and detergent–TSP is most effective but can be hard to find. This might be #2 in some cases.
4. Dry thoroughly.
At this point the inside of the tank should be clean shiny metal. Dent removal, patching, etc. now.
4. Coat inside with POR-15. This will stop small leaks and future corrosion, but ONLY if put on clean metal.
All this, obviously, is a pain in the ass, and takes days, but should leave you with the practical equivalent of a new tank.