Make galvanized metal appear aged by soaking it in white vinegar overnight or until the desired effect is achieved. Iron paint can also be applied to bare metal to make an object appear rusty.
To age or "antique" galvanized metal, soak it in full-strength white vinegar for as long as it takes to achieve the desired level of antiquing. Alternatively, spray or dip the item in a 3:1 solution of apple cider vinegar and a non-iodized salt, such as sea salt. The latter produces a patina affect on gold or yellow metals.
To make an unpainted metal object appear rusty, apply an iron reactive paint. Brush the entire object, or just dab it to the areas where rust is desired. After the iron reactive paint dries, apply a coat of rust activator. A popular brand of both iron reactive paint and rust activator is Metal Masters. After applying the rust activator, results begin to appear within about an hour. More rust continues to develop overnight or until you apply a clear topcoat sealer. This method works well to create a faux rusty look because the rust only forms on the iron reactive paint and not the metal itself. The metal object underneath remains free of corrosion.