Select a hammer that is approximately the same size as the dent. This is important to ensure a consistent change in the shape of the metal of the French horn.
Apply a sheet of teflon to the hammer head to avoid causing damage or removing laquer from the French horn.
Use your right hand to hold the hammer and your left hand to hold the French horn firmly against a clean cloth. Apply a gently amount of force in a tapping motion of about one tap every two seconds to slowly form the dent back to the original shape, moving from the outside of the dent inward. French horns have large bells, and this is where most dents occur. Support the bell carefully to avoid bending the tubing. Having another person hold the horn while you hammer is the ideal situation.
Polish the French horn using a brass or silver polishing cloth, depending on the material of the French horn. Polish until the instrument is clean and shines. Rub in a circular motion to clean the French horn evenly.
Select a magnetic ball bearing that fits easily inside the French horn tubing with the dent. Make sure there is room for the ball-bearing to move around. The ball-bearing should be approximately three-quarters the size of the tubing.
Drop the ball-bearing into tube and rotate the horn until the ball-bearing is in close proximity to the dent.
Place a thin sheet of cloth on the outside of the horn covering the dent to prevent damage to the instrument finish. Roll the ball-bearing by moving the magnet over the dent several times. Move very slowly, allowing the ball-bearing to depress against the dent through its attraction to the magnet.