Care & Maintenance: Cleaning, Repair & Restoration
The rule of thumb about cleaning is . . . . . don't! Locks are of most value to collectors in their as-found condition. Removal of grease and dirt is fine, but removal of any original finish is not fine. Rust requires some discretion. Many dealers will remove rust to make them more "marketable", and heavy rust is generally objectionable. However, in the case of very old or ancient locks, an overall patina of the rust of ages is something of a badge of authenticity. Under no circumstances should patinas (corrosion) be removed from locks and keys of brass or bronze. It's very easy to destroy value in this way. Repainted locks look unnatural, and many collectors avoid them.
Locks are very durable, and require little protection. If you live in a very humid climate, you might consider a dehumidifier to prevent rusting of iron/steel locks with bare metal surfaces. An inexpensive alternative is simply a light coating of oil, but this could become a problem with a large collection.
Repairs should be left to the services of a locksmith, which generally means that only very valuable locks should be repaired! Many, many locks have been the victims of would-be restorers who mill off rivets to open the case, and end up ruining the lock. It is difficult or impossible to open a lock case and re-close it without leaving evidence.