A good exterior 100% acrylic paint is fine for metal as long as it is properly primed. Many garage doors are either factory primed or finished. With both instances it is recommended that a primer be used before painting.
There are many exterior primers on the market that will work great. I recommend a fast drying universal type of primer like Zinsser 123 (acrylic )or Zinsser Cover Stain (oil based).
At this point you need to remove all loose paint and spot prime with one of the above primers, then repaint the garage door. You can try touching up the door before completely repainting.