[Benson]

It is possible that the rust on the fins is from a small leak in the core that is letting rusty water from inside the radiator / engine block because antifreeze is NOT being used.

Even though the radiator is new they CAN have leaks due to defects or leaks from radiator cap.

Water in a radiator is the first to freeze long before the water in the engine block freezes.

Even though the radiator is new they CAN have leaks due to water freezing in the radiator core if antifreeze in not being used.