Can you paint over peeling paint on your outside garage doors?

Personally, I suggest to do it just like you would with walls. First, scrape the loose paint using a wire brush (gently, as you don't want to harm the door). Do not use a paint scraper, as you might create holes in the panels and you don't want that. I can't stress how gently you need to work, but I think you got it by now.

Afterwards, use one or two layers of paint. Make sure it goes with the material of your door. A material that works with steel does not necessarily work with wood. Speaking of wooden doors, you might want to add a protective coating at the end to prevent rotting.