I had a customer bring in a sentimental cheap watch with the gold
plating worn off the back.
Been there, done that. About the only thing I've been able to come up
with is to clean the watch case as thoroughly as possible and paint
on a thin layer of good epoxy on the back. Sort of a plastic
coating. Trying to re-plate that base metal, especially after it's
pitted and corroded, is pretty much impossible, and not a very
lasting solution anyway.
David L. Huffman