The real secret of Hot Dip Galvanising is Sacrificial Protection. Zinc corrodes in preference to steel and also sacrifices itself to protect any exposed steel. This is also known as Cathodic protection and ensures that areas of minor damage to galvanized coatings do not become a problem.
To illustrate this, we galvanized half this farm gate and left the other half exposed to the elements. You can clearly see how the rust is halted where it meets the galvanizing and no sideways creep occurs.