Here is a legitimate use of a coated postal card. These cards are somewhat scarce, and desirable.
There were generally two coating methods used to cover an earlier preprinting. One coating was an enamel, or "clay" coating, similar to todays convenient "white-out" fluid. Others included a bronze, silver, or gold coating, which reminds me of the scratch-off substance used on instant lottery tickets, albeit the coating would have been much harder to allow the surface to be printed or written upon.
The preprinting beneath the "gold" coating indicates that this card was formerly addressed to one Oliver Sollitt, General Contractor at the Hartford Bldg, 140 Dearborn St. in Chicago.
Place mouse on image to see the reverse side of the card.