Place 4 tbsp. of baking soda in a bowl. Drizzle in lemon juice and mix until you form a paste. If you're using it on large or multiple fixtures, you might need to increase the amount you use.
Dab the polishing cloth in the paste.
Rub the cloth over your bathroom fixtures in circular motions to apply the paste and break up the patina. Continue until it begins to wear away, revealing the bright finish underneath.
Allow the paste to rest on the fixture for 30 minutes.
Dip your sponge in lukewarm water and wring until it's damp.
Remove the paste from the metal fixtures with the sponge. Dry them completely to stave off the formation of new patina.