A piano is a major investment. One can cost more than a car, and it needs to be tuned regularly to maintain its beautiful pitch. Most services that are required in order to maintain a piano are simply too complicated for an owner to deal with himself. However, issues such as scratches in a piano's black lacquer surface finish can easily be resolved at home and can save the owner thousands of dollars in repair fees and expenses.
Buff the scratched area with the steel wool. Try to ensure that there are no rough spots or edges on the repair site.
Fill the plastic bowl 30 percent with thinner and 70 percent with lacquer. Fill the spray gun with this mixture.
Adjust the air regulator on the compressor to 30 PSI and practice spraying the scrap wood with the lacquer-thinner mixture. Try to get even flow and coverage from the gun. If the lacquer isn't spraying easily or is not covering sufficiently, add some more thinner to the lacquer in the gun. The percentage of lacquer to thinner should never exceed 50/50.
Spray the scratched area of the piano, making sure to not create too thick of a layer of lacquer.
Let the area dry for two hours and then lightly buff the area with the steel wool until it is smooth and there are no more blemishes.
Mix a new batch of lacquer that is in a ratio of two parts lacquer and one part thinner. Empty the spray gun and fill it with this new mixture. Apply a coat of the lacquer over the area being repaired and then wait for five minutes. After five minutes have passed, add another layer of lacquer.
Allow the repair site to dry for at least two days before continuing any further.
Buff the repair area lightly with steel wool to remove any dust that may have dried into the surface of the lacquer.
Wet a piece of 1000-grit sandpaper and lightly sand over the repair area. Repeat the same thing with the 1500-grit sandpaper and the 2000-grit sandpaper.
Polish the surface with 4F pumice, using water as a lubricant, and a cotton rag. Use a clean cotton rag to wipe the area clean.
Polish the surface again, but this time use rottenstone and water as a lubricant. Buff the area clean with a cotton rag, and then polish the surface again using rottenstone and mineral oil. Using another cotton rag, buff the area clean.
Wipe the surface down with mineral spirits to remove any dirt or residue that remains.
Never place drinks or other items directly on the lacquer finish.
Scratches can be avoided by always wiping the surface down with a wet cloth prior to using a dry one.
Only repair scratches in temperatures above 65 degrees Fahrenheit, or the lacquer can become cloudy when it hardens.