Apply a 1 part salt, 2 part water solution to the sheet metal, and let the solution sit for one or two weeks. Reapply the solution as needed. The patina on sheet metal can be hastened by washing the material with the solution. All metals oxidize, but many metals have a very characteristic patina. Steel creates a reddish rust, copper and bronze have a brown or greenish patina, and silver has a white or gray tarnish.
Firmly rub sandpaper, a sander, or a rasp on the sheet metal to wear away layers of oxidation. Repeat as necessary. The wear can be a pattern, writing, image or ornament. Materials such as copper and silver oxidize in different colors when they are exposed to different chemical elements in the air. Take advantage of this phenomenon by re-oxidizing the sheet metal in another environment after the surface patina has been sanded and worn.
Create pits, indentions and divots in the surface of the stainless steel, brass or aluminum sheet metal by continuosly running the tool over a surface with an orbital sander or drill and wire brush wheel. Continue to run the sander or wire brush wheel repetitively over the surface to create deeper and deeper marks in the surface. Experiment with brushing in different directions to create varying surface qualities on the metal. Furthermore, wear repetitive patterns or composed images into the sheet metal surface by running the sanding tool over the sheet metal's surface.
Plate the articulated surface with chrome at a local metal fabricator. The shiny, reflective chrome plating highlights the differentiation in the sheet metal's surface.