Iron door knockers are probably the most likely to show signs of weathering. If you choose a door knocker made of iron material then you should treat it with chemicals that will slow or halt the oxidation process. Check your door knocker periodically to spot rusting. If the iron begins to rust then sand the affected area and paint the door knocker with a rust retardant paint to further protect your door knocker.
It is important to catch rust or other damage early so it doesn't spread and affect your door.