Determine what type of finish your French horn has so you can match the finish when relacquering. If you are unsure, you should consult a professional at a music store.
Completely disassemble the French horn. Remove the mouthpiece, slides and pistons. Set them in a safe location for later.
Pour hot water into a large tub or sink. Set the French horn in the water for up to four hours. If the lacquer was chipping before, most of it will be removed in this process. Rub the horn with a lint-free microfiber cloth after taking it out of the water.
Remove any remaining lacquer on the horn with a lacquer remover using a lint-free microfiber cloth.
Thoroughly dry off the horn's exterior with the microfiber cloth and allow the rest of the horn to air dry for several hours.
Cover the work surface with a drop cloth or a plastic sheet. Spray or brush the lacquer on the outside of the horn. Do not lacquer the pistons, mouthpiece or slides since they will go inside other parts of the instrument.
Wait up to 24 hours for the lacquer to dry. If you are applying multiple coats, wait 24 hours between each coat. Slide the mouthpiece into the mouth pipe, push the pistons into the valve covers and push each slide into the pistons to reassemble the horn.