The origins of japanned metal are actually in China, though the Japanese and Indians also adopted and contributed to the art. It's called japanning because the Japanese, not the Chinese, were the people the Dutch came into contact with in the 16th century.
Western styles of japanning date from the 17th century, when Europeans learned the technique of applying layers of shellac or varnish to metal. This imitated the authentic japanning found in Asia, which was difficult to obtain for European consumers. In addition to beautification, another reason for japanning metal was that it made it resistant to rust. One of the centers of japanning in Europe was in the town of Pontypool in Wales.