Copper plating and nickel over copper was sometimes done before WW2 on racing motor cycle frames so that cracks could be easily seen. There is a good example at the Brooklands Museum very near to the collection of bicycles. If the polishing is missed out, plating makes a good key for paint. If any makers did use paint over plating, I would expect them to have mentioned it in their advertising. Most of them were not backward in coming forward! Stuart Tallack