Steel surfaces are ill-suited for paint adhesion, because they are slick and nonporous. If the paint is peeling from your steel garage door, the likely culprit is poor surface preparation. Steel garage doors first require a base coat of an acidic primer. If you want to fix the peeling paint, your only real option is to strip the failing areas bare and touch them up with fresh primer and paint. Know the proper materials to use and the appropriate ways to employ them, or you will see a recurrence.
Clean the steel garage door, using a pressure washer, equipped with a low-pressure tip. Wait two to three hours for the door to dry.
Scrape loose the failing paint, using a plastic putty knife.
Smooth the edges of the peeling paint so they conform to the rest of the door. This means sanding them with 120-grit sandpaper.
Coat exposed areas of bare steel with galvanised metal etching primer, using a two- to three-inch paintbrush. Wait four hours for the etching primer to cure. Wash the brush with water.
Touch up the primed areas with acrylic latex paint, using the clean paintbrush. Wait two hours for the garage door to dry. Apply more paint if the etching primer bleeds through.
Never apply paint directly over bare metal portions of a steel garage door, or the finish will peel. Don't use ordinary latex, acrylic or oil primers on exposed bare steel, or the finish will ultimately flake.