I like using Evapo-Rust to deal with rusty areas because it isn’t acidic and it doesn’t eat away at the good metal.
Cub tanks are really thin. If you have rust some of the metal is already gone. An acid is effective in removing the rust but it removes good metal as well. Evaporust only removes the rust and a pint can should fill your tank. Cheap compared to finding another tank. As you have had gas in the tank you need to remove the oil left in it first. Evaporust is water base and the oil can keep it from getting to the rust.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.