In the real world, round aircraft instruments are often bordered by a black, square panel that's mounted to the instrument panel (as seen in the F-14 above). I have long tried to paint this, and I have long failed to achieve a sharp border. Instrument panels are too small to mask and my hands too unsteady to paint a straight line on something that small.
The perfectionist in me is always trying to come up with ways to make things look more realistic, while the slacker in me doesn't want to have to do any extra work for it. The two got together and came up with this.
To make the black border, simply cut a black piece of decal paper to the size of the instrument (I use calipers to measure, but you can eyeball it just as well). Place this piece of decal film over the instrument (red arrow), then melt it down with the setting solution of your choice (blue arrow). After it dries, drybrush as usual.
For a row of instruments, merely cover the entire row at once, rather than each one individually. No masking, and razor sharp lines on the edges every time. Precision isn't required as you can always touch up trouble spots with black paint applied with a toothpick.
Anyway, this method has been working for me, so I thought I'd share it with you.