In this day and age of modern firearms coatings, it’s easy to forget that older guns are not as rust-resistant as those made of stainless steel or treated with the latest wonder finishes. This excellent article that deals with caring for older metal finishes was inspired by a shooter who discovered that a Mosin-Nagant he’d neglected in storage for a year or so had started sprouting ugly orange patches of oxidation:
European firearms of the 19th and 20th Centuries, and many contemporary sporting guns, tend to be rust-blued with occasional bare parts (like the Mosin bolt) or, especially in German arms, heat-strawed or flame-blued small parts. In fact, most 20th Century Eurasian Communist AK and SKS rifles are finished similarly. This is a very beautiful finish and probably met 19th Century expectations for durability. By the mid-20th Century, phosphate finishes were widely in use and are commonly seen on US arms (“Parkerizing”) and late-war German arms. But even Parkerizing has its roots in the early 20th Century; while it should still be used in restoring those historic arms for which it is appropriate, the moving finger, having writ, has moved on, and modern electrochemical treatments and surface coatings are superior.
Keep an eye on older guns you may have. Oil them periodically, try to store them in a dry environment, and inspect them regularly to make sure the rust monster hasn’t moved in and taken up residence in your gun safe.