Brass, an alloy of copper and zinc, tarnishes quickly when exposed to air. According to Martha Stewart, your brass door knocker is probably coated with a protective lacquer which slows down this process. Eventually, though, the fragile lacquer wears off with exposure to the elements. Cleaning untarnished brass is a simple process. If your brass is tarnished, though, you'll need to remove the tarnish and apply a new coat of lacquer
Clean your untarnished brass door knocker regularly with a squirt of mild dishwashing soap mixed with tepid water. Hot water can damage the lacquer. Use a soft cloth to wipe away dust and debris.
Remove lacquer from tarnished door knockers by applying a coating of acetone on a cloth. Wait five minutes and remove the stripper with a brush. Wear protective gloves for this step. Rinse and dry the brass door knocker thoroughly.
Polish your brass door knocker using a commercial brass polishing product. Apply the polish with a soft white cloth (a diaper or old T-shirt) and rub in a circular motion. Wrap a cuticle stick or old toothbrush in the cloth to get into ornate, hard to reach spots. Wear gloves to prevent fingerprints . Rinse the polish off and dry thoroughly with a clean, dry cloth.
Spray a spray-on lacquer on your brass door knocker. Spray lightly, making sure it doesn't drip. Apply several coats, allowing the product to dry between coats. Tape paper or plastic to the door to protect it from the lacquer.
Dip a clean cloth in car wax paste. Apply the paste to your brass door knocker in a circular motion. Wait five minutes. Buff with another cloth until the paste is smooth and dry. This will protect your door knocker from tarnishing in the future.
Clean and polish your brass door knocker on a clear, windless day. Consult a conservator about historical or highly valuable brass door knockers.
Use care when handling acetone and polish. These products can irritate the skin. Keep them out of children's reach.