I have a laquered brass door knocker that has been exposed to weather for many years. What is the best way to clean it without damaging it?
Brass begins to develop a patina within hours of being exposed to the elements. Brass is a alloy of copper and zinc, and although when cleaned and polished it can have a brilliant shine, if it is not protected it can tarnish and actually pit the surface. Most brass architectural hardware, such as your door knocker have a protective coating applied to them. These coatings prevent immediate damage, however almost all fail after a few years of exposure. This said, there are some new technologies in coatings which are much more durable than those applied even a few years ago.
If your door knocker has had one of these coatings, you will be able to tell if it appears the surface is protected in some areas, yet damaged in others. Some of these coatings appear to flake off of the protected surface. You can try cleaning the piece yourself, however if the damage has pitted the surface, or if the protective finish is only partially intact, you will likely not be very happy with your efforts.
If the surface is completely intact, washing with a mild detergent (such as dish soap) is enough to remove the grime that accumulates over the years. It is a good idea to protect the lacquered surface with a couple of coats of high quality paste wax. This will prevent the elements from damaging the lacquer finish itself, and effectively provides another barrier to the brass itself.
If the piece does not appear to have a protective coating, you can try a metal polish. Metal polishes have a very fine pumice suspended by detergents and sometimes a wax. You apply the cleaning polish, and rub, and rub, and rub. And finally to polish the piece, you buff some more with a clean dry cotton cloth.
Once the piece is polished, you need to protect it with a lacquer that is suitable for outside elements. I have also heard of using several coats of automotive paste wax, to prevent elements from reaching the brass, although you cannot control the glossiness of the piece when using wax (lacquer’s at least come in different gloss levels).
For serious damage, a professional will take your piece, and possibly sandblast it to remove pitting and the remaining protective coating. They will then polish and apply a protective coating.