I put some transparent acrylic paint sold als acrylic finish (used for art paintings) on it. On my second watch I used white arcylic paint since it is easier to see if it's still on the watch. If you paint carefully the transparent finish cannot be seen.
I've used some transparent varnish, but I think I'll replace with black acrylic paint for the same reason you used white: to be sure that it's there. I think it's holding up, but I'm not sure and I also think I'm getting some corrosion so better safe than sorry.
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I put some transparent acrylic paint sold als acrylic finish (used for art paintings) on it. On my second watch I used white arcylic paint since it is easier to see if it's still on the watch. If you paint carefully the transparent finish cannot be seen.