I don't think it makes any difference whether metal or wood in this case.
You could of course strip it right down but from what you say it doesn't
seem necessary. What I did with my own garage door is first make sure the
old paint is thoroughly clean - using a softish broom and Mangers in a large
bucket, then rinse liberally with water. (I find a broom ideal because it
gets into any grooves as well as being practical on such a large surface.)
Obviously allow to dry thoroughly then sand all over the door very well,
making sure there's absolutely no loose paint. After that use the undercoat
and topcoat. I stand to be corrected but I think you only need primer if
the bare wood or metal is visible.
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