Plating was the practice of coating the outer surface of something with a metallic substance.
Zinc plating was used by Humans in the early 20th century. In 1930, when James T. Kirk suggested Spock build a device to enable him to access his tricorder scans of the Guardian of Forever, Spock felt it couldn't be accomplished in "this zinc-plated vacuum-tubed culture". (TOS: "The City on the Edge of Forever")
Latinum plating was considered a luxury. In 2369, Quark asked Odo whether he ever desired material possessions when the Changeling expressed his confusion at some people's obsession with attaining them, asking whether he ever desired to own anything. Among other items he suggested was a latinum-plated bucket to sleep in, as opposed to his more humble one. (DS9: "Q-Less")
The waste extraction fixtures in the Nagal Residence were plated with latinum. In 2375, Nog mentioned he had heard this to Quark. As he was anticipating being named the new Grand Nagus at the time, Quark stated his intentions to replace them with solid latinum fixtures. (DS9: "The Dogs of War")