Vintage keys and other antique metal objects develop a patina over time. While brass keys may develop a desirable mellow hue, other keys corrode and become rusty. Rust may be removed from older keys, but before undertaking this task, weigh the risks. A very old key may be brittle and break with abrasion. In the case of sturdier keys, patina and rust may be desirable. Identify the age, desirability and scarcity of keys before attempting to clean.
Sort and inspect keys. Set brass keys aside, as patina may be desirable to collectors.
Pour distilled white vinegar into a non-metal container. A clear glass vase allows observation. Place a test key in the receptacle.
Allow the key to sit for several hours. Remove key and rinse. If rust is lifting, gently rub the key with a wire brush to loosen debris. Place key back in the vinegar, where it can be left for several days. If the strength of the metal is unknown, however, periodically check the key.
Remove the finished key from the vinegar. If the desired result has been achieved or is close to being achieved, thoroughly rinse the key. Vinegar will continue to eat away at rust and debris unless rinsed. Dry the key thoroughly. If metal feels stripped or bare, polish the key with oil or brass or silver polish.
Err on the side of caution when determining soak time. Vinegar removes rust, but when metal is soaked too long, the acid causes deterioration, making the key's surface bumpy. Do not heat vinegar as heat accelerates deterioration.