Because the metallic makeup of each coin is different they will react differently in the atmosphere over time. Although destructive changes may happen to unprotected coins sometimes they are stable and require no protective coatings or sealants. Unfortunately, it is hard to determine which ones will benefit from further processing or not. In the worst of cases a coin may crumble into dust after cleaning either because of corrosion that was already present or the development of "bronze disease". Bronze disease is a hard-to-treat type of corrosion that appears as pale greenish and powdery "warts". Corrosion spots nearly always create irreparable damage to a coin's surface.

The easiest, and sometimes most effective, way to preserve a coin is to apply a thin coat of oil over it. The oil serves as a water- and air-proof way of sealing it. The main problem with oiling is that it needs to be re-applied every now and then as the coat slowly evaporates. An oily film is not always appreciated either. The recommended type to use is mineral oil.

Another time-honored way of preserving coins has been to coat them in a thin layer of wax. The advantages are that it is as effective as oil in preventing corrosion but doesn't need to be reapplied as its application leaves a permanent film. Waxing also can improve the contrast of a coin by buffing. However, wax is difficult to remove should you decide later on to strip it.

Also permanent, easier and cheaper to apply are acrylics. Acrylics are liquified plastics that harden on contact. When used to seal coins they can give either a glossy or matte look. While the glossy look is attractive to some, others find it (as well as buffed wax) too unnatural. A matte acrylic leaves behind an almost invisible film that approximates the look of an untreated coin.

Coins treated with sealants nearly always tend to look a little darker.