Anyone who has watched the popular public television broadcast "Antiques Road Show" knows that many fine antiques have been ruined by over-zealous cleaning. An article published by HowToCleanStuff.net reminds us that "antique metals develop a certain look over time due to patina and oxidation [and] it would be detrimental to the value to try an alter the color in any way." For this reason, only gentle methods are recommended for cleaning an antique metal sign.
Mix a small amount, no more than a teaspoon, of a mild dish soap or a natural, plant-based cleaner, with a gallon of water.
Moisten the cloth with the soap mixture and wring it out well. Gently wipe away dust and dirt from your metal sign.
Use a clean cloth and clean water to wipe off any soapy residue.
Dry your metal sign with another clean cloth. Make sure to dry the sign completely. Oxidation or water marks may occur if it is left moist.
If there is soil or stains that can not be cleaned by a gentle washing, consult an antique expert before using any type of metal cleaner on your antique sign. Further cleaning may be possible once the type of metal and value of the item has been determined.