Paint will not adhere to a metal hinge unless it is specially conditioned to accept the paint. Unlike wood, metal is not porous and cannot soak up the adhesives in paint. Before you apply paint to a metal hinge, you must abrade it to promote adhesion. Unfortunately metal is too hard and durable for friction-based abrasion based on sanding. Instead, you must apply a special kind of primer to the surface that etches the metal and gives the paint something to adhere to.
Remove the metal hinges with a screwdriver, and take them to a well-ventilated area.
Clean the hinges using mineral spirits and a rag. Allow 15 minutes for the mineral spirits to evaporate before continuing.
Lay the metal hinges on top of a fabric drop cloth.
Put on latex gloves.
Apply a very thin coat of etching primer to the metal hinges. Apply only short, intermittent bursts. Do not spray a steady, consistent stream as this may lead to runs in the finish.
Allow the etching primer to cure for four hours. Open and close the hinges every 30 minutes during the dry time to prevent them from sticking.
Apply two coats of oil-based spray paint to the metal hinges in the same manner as you did the primer. Allow two hours of dry time between each coat.
Allow the paint to cure for four hours before you reattach the hinges.