Paint can lose its adhesion to a galvanized metal substrate. 
What Causes It?
Poor galvanized metal adhesion can occur:
When a surface has not been properly prepared.
When a primer has not been applied before an oil-based paint is used.
When baked-on enamel finishes or glossy surfaces have not been sanded before painting.
How to Solve ItFirst, use a wire brush to remove any rust on your metal surface. Then, apply a corrosion-resistant acrylic latex primer—one coat is usually sufficient.
With galvanized metal that is new or has been painted before and is rust-free, clean the surface to remove all fabricating oils.
Then, paint it with a top-quality acrylic latex paint without applying a primer.
With unpainted galvanized metal, however, always use a metal primer before you apply an oil-based or vinyl latex topcoat.
Note: Images provided by The Rohm & Hass Paint Quality Institute.