Brass will automatically start to tarnish if exposed to oxygen. The only way to prevent polished brass from tarnishing is to coat it with a layer of clear sealer that will prevent air from reaching the brass itself. Many brass surfaces come with this finish originally, but it can wear down over time. Refresh the coating every few years to keep the brass shiny and like-new without frequent polishing. A clear polyurethane sealer will protect the brass from oxygen exposure without causing discoloration.
Apply a small amount of paint thinner to a soft cloth. Rub the thinner over the surface of the brass to remove any existing coatings. Wipe the brass surface with a clean, damp cloth to remove any residue. Allow the brass to dry overnight.
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Polish the brass with a mixture of lemon juice and salt. Add enough salt so that it does not dissolve in the lemon juice. Wash the brass with soap and water when it is shiny.
Paint a coating of polyurethane onto the brass with a paintbrush. Allow the coating to dry two hours. Apply a total of four coats of sealer over the brass. Allow the brass to dry for 24 hours before moving or touching.