These photos  might help you to understand how iron is coated by zinc:

* Left: By dipping into hydrochloric acid the surface of the iron
   is cleaned from rust and other impurities.
* Middle: After drying the materials are dipped into molten zinc.
* Right: The iron of the can is fully covered by a thin zinc layer.
Iron is no longer in contact with air and water, rusting is not possible.