How to repaint a metal door and keep the paint from scratching off?
Priming and Painting Galvanized Metal, condensed from KILZ.com, other manufacturers also make specific paints and primers for galvanized metal.

The galvanizing process, which is designed to prevent rust, leaves an oily film that can prevent coating adhesion. The zinc in galvanized metal can produce a milky “white rust” (which is common when it has weathered) that must be removed with a stiff brush or abrasive pad, prior to coating. It’s necessary to remove the oily film that can prevent coating adhesion with a water-based cleaner/degreaser. 

Prime the surface with a water-based primer. Since oil-based primers can interact with the zinc in galvanized metal, causing premature peeling, always use a water-based primer when painting galvanized metal. KILZ® 2 Primer is an excellent choice when priming and painting exterior galvanized metal. It will grip to the metal surface and provide a better surface for the paint to adhere.

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