Painting Metal Garage Doors:

Most metal garage doors come with a very hard Kynar finish over steel that has been treated with zinc galvanizing.
If the door shows no sign of oxides then all it takes is a cleaning followed by a bonding primer like XIM UMA and then a finish coat of top quality acrylic house paint (I prefer a semi-gloss for clean-ability).

Zinc Oxide

If the door has oxides then , we need to add some steps.
Because the door has been treated with zinc, the first oxides to show are a dull gray zinc oxide….if this is allowed to continue, the zinc will be depleted and you will get down to the underlying steel and you will see the more problematic red oxide.
They key is the lightly sand to remove excessive oxides and prime areas where the zinc oxides showed themselves with a cold galvanize zinc primer to replenish the protection of the galvanized surface.
Where red rust has appeared, I recommend a phosphoric acid treatment primer that will convert the red rust to a less reactive compound.
Red Rust/Iron Oxide
Now the whole door can be primed with the bonding primer and finished with acrylic house paint…properly maintained, a metal door can last a very long time.