Assuming it's a metal garage door, you MUST sand it down. If not, it will peel, and you will not, not like that - as scraping loose paint off is no fun.
You would have to prime any bare metal, probably with a special metals primer.
You would then need to apply undercoat (oil-based best for metal IMO).
Then topcoat. Oil-based gloss is best IMO, as will give a nice shiny hardwearing finish.
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