I had some ideas since painting mine...
if you repaint the inside of the numbers and let it dry a day or so, you can put vaseline inside the numbers to cover that paint.
Then, after the black area has been sanded, you just spray it a few times.
Once the black dries real good, you wash the vaseline out with warm soapy water.
Then get some 2000 grit sandpaper flat, and turn the emblem number side down and sand it and it will shine like new chrome.
I guess anything that will "seal" off the paint inside the numbers would work.