I mix up some matte black 2-part epoxy paint and dab it onto (into) the scratch or chip with a small instrument like a toothpick. Wait until it's partly dry (5-10 minutes) and start tapping the paint lightly with a bare finger until it levels nicely inside the blemish. If you get any paint on your finger, it hasn't cured enough and you'll have to wait longer before tapping on the paint. Let the paint cure for another 5-10 minutes, then lightly rub it with a wet magic eraser until it's level with the original paint around it. At this point, the filled crack is smooth but dull, so once again start tapping the surface with a bare finger (more pressure this time, and may require pressing) to achieve the matte look again. This method works great.