If you simply apply primer and paint to a galvanised metal surface, like you would to most other surfaces, the paint will begin to peel away after just a little bit of time. The reason for this is because in the galvanising process, the metal develops a thin protective layer of oil that prevents good paint adhesion. Weathered galvanised metal also develops a thin coating of white rust that will prevent proper paint adhesion. Both the oil and the rust need to be removed before you can paint your galvanised metal garage doors.
Scrub the surface of the garage door with a wire brush. Go over the entire door until all the white rust is removed. If your galvanised metal garage door is brand new, you will not have to do this, since the rust hasn't formed yet.
Clean the garage door's surface with a water-based cleaner and a sponge. Wait for the surface to dry before continuing.
Dampen the cloth with white vinegar and scrub down the entire surface of the garage door. Reapply the vinegar to the cloth as necessary.
Apply primer to the garage door, using long strokes. Continue until the entire door is coated in primer.
Wait a number of minutes or hour according to the direction on the can of primer. Drying time for latex primer will be about an hour, but it can vary wildly based on the brand of primer, temperature and humidity.
Apply the latex paint in the same manner as the primer. Wait until the paint is dry, according to the drying times on the can, and apply a second coat, if you desire.