Because brass is a slick, nonporous surface, you must abrade it (roughen the surface) before any paint will stick. Brass is different from other types of metals, so you will need to use a specific type of etching product. Otherewise, you may accidentally cause a chemical reaction that could release toxic fumes into the air.
Remove the brass door handles using a screwdriver.
Place the brass handles on top of a heavy-duty fabric dropcloth.
Clean the door handles using white spirit. Wait 10 minutes for the spirits to evaporate before moving on.
Abrade the brass handles to promote better paint adhesion using PCB etch. Follow the directions on the specific type of PCB etch you purchased.
Rinse the brass door handles thoroughly and allow them to dry.
Spray an oil-based primer onto the brass handles. Apply in brief, intermittent bursts. Do not saturate the handles as this may lead to sagging in the finish coat. Allow the primer to dry for two hours.
Apply a coat of oil-based spray paint to the primed brass handles in the same way you applied the primer.
You can find PCB etch at most home improvement or paint stores.
Harsh acids are generally employed to abrade steel and other types of metal. Do not use acid to etch brass as this will release toxic fumes into the air.