I just did mine. You can get pinstripe tape to mask off the dial line then just paint the dial black, not too thick or the paint will build up along the tape.
I use fuel proof black paint to fill in the numbers, then wipe the face with solid cardboard, like a cereal box, to remove the excess paint. I cut a thin strip of clear tape and placed it across the middle of a wire brushed face plate. I sprayed Plasticoat flat or semi gloss balck, then after the paint sets for a few minutes to let most of the solvents escape, I remove the tape. This leaves a nice thin line. I'll see if I have a picture of mine, which I did about 10 years ago. Most of the time I don't have to do the dial, because it's in good original condition. I only recall painting one dial.
I tried Testor's fuel proof dope (which worked great on my carb) on my gas gauge, but it seemed to have poor adhesion to the aluminum (or pot metal or whatever), and I was worried about it flaking off. I ended up using Eastwood Ultra Chassis Black spray, which is Epoxy-based. It has held up fine for two years so far: