You can repair dented lacquer finish without sanding off all the lacquer. Dented musical instruments are heavily coated with nitrocellulose lacquer which is one of the hardest of all lacquers. To sand them down to remove a dent would be counterproductive, and you could possibly ruin the instrument. Some pieces of fine furniture have small pieces or delicate decorative elements that make individual dent repair the only way to go.
Arrange the piece you are working on in such as way that wet lacquer will not drip off. The lacquer will need to set into the dent.
Stick masking tape in short strips around the perimeter of the dent. Dip the cotton swab into the lacquer. Get a small drip of lacquer on the tip of the swab.
Apply the drip of lacquer carefully to the dent. Carefully fill the dent with the lacquer. Try to put enough lacquer in the dent so that the surface of the lacquer is actually raised up higher than the surface of the dent. Wait one hour for the lacquer to dry.
Apply another drip of lacquer to the dent. By this time the lacquer will have shrunk into the dent forming a small cavity. Fill the cavity with lacquer. Let the lacquer dry for one hour.
Apply more lacquer to the dent a drop at a time and let it dry until the lacquer is slightly raised up above the dent when dry.
Sand the dent flush by hand with 400 grit sandpaper. Remove the tape.
Apply a small amount of polishing compound to the dent. Use a circular motion to polish the area around where you fixed the dent. Wipe off the compound. If you can still see a dull area where you polished, add more polishing compound and polish until it is blended.
You can pick up polishing compound at a local auto parts store. Buy the finest grit they have. Any type of lacquer will do, but try to purchase high-gloss.
Don't let any lacquer drip onto the finish other than into the dent. Lacquer will melt other lacquers.