Tuesday, June 22, 2021

Metal Finishes

When choosing metal items that will be exposed to wear or the elements, whether lawn furniture, firearms, or decorative items, the finish can have a considerable impact on longevity.

Metal finishes come in a variety of categories with different pros and cons in protective qualities, care requirements, and aesthetics. In this column I'll review some of the more commonly encountered methods of finishing. As I am a gun nut firearms aficionado, I will be using firearms as my examples, even though many of these finishes can be applied to other metal objects. 

One of the oldest methods of protecting metal is case hardening. Also called surface hardening, it’s more of a metal treatment than an actual finish; it provides an extremely hard surface while maintaining a softer core, and has been used on everything from self-tapping screws to crankshafts. Color case hardening is a subcategory usually found on firearms that adds an attractive color pattern in addition to protection. Since surface hardening is very thin, it can be worn away by continuous abrasion; however, due to that same hardness, it resists wear quite well. The pattern of color case hardening can fade over time, but the hardness will remain.

Color case hardened frame on the author's
reproduction Single Action Army

Another long-used finish is bluing, also called rust bluing. As the name implies, bluing is a controlled form of rust. Various processes are used to convert the surface of the metal to black oxide or magnetite. While bluing can provide good rust resistance, it has little wear resistance and can scratch through fairly easily. Other than as a traditional firearm finish, bluing is more commonly found used on decorative items these days.

Blued finish on the author's S&W Model 27-2

Both nickel and chrome plating have been used for many years as wear-resistant, corrosion-protective coatings for both metal and other materials such as plastic. These finishes are generally applied through either a chemical or electrolytic process. Marine equipment or other items likely to encounter a corrosive environment are frequently chrome plated to prolong their usable life. Improperly done, nickel and chrome plating can form bubbles where the chrome or nickel does not adhere to the underlying material, which often leads to peeling of the finish.

One of the author's nickel-plated pocket pistols

Another finish primarily used for firearms and other military equipment is Parkerizing. Originally patented in the 1860s, Parkerizing is a very corrosion- and wear-resistant finish, but is not commonly found on decorative items. In this process, zinc (chemical symbol Zn), manganese (chemical symbol Mn), or iron (chemical symbol Fe) are placed with the parts in a heated phosphoric acid solution resulting in a medium to dark grey textured finish.

Parkerized receiver of the author's M1 Garand

Stoving is a traditional metal finish most commonly found on grills and other high heat items, hence the name. It’s a type of baked-on enamel paint that provides an extremely tough corrosion and wear resistant finish. Traditionally coming in satin or matte black, modern high temperature stoving paints can be bought in a wide array of colors. The various epoxy-based coatings, such as Duracoat, Ceracoat, and Norrell’s are descendants of stoving and, if anything, can provide an even tougher finish. Powder coating, though applied dry, can also be considered part of this category.

Stoving applied to the C7 Upper receiver of the author's rifle

Water transfer printing, also known as hydro dipping, is a decorative coating process used on everything from car parts to personal protective equipment. Unlike most of the other finishes or coatings mentioned, hydro dipping is commonly applied in a pattern, such as camouflage, and can be applied to nearly any material. While attractive, hydro dipped finishes are mostly applied over a more weather resistant finish if the item is to be exposed to the elements. Like bluing, hydro dipping is not usually very wear-resistant and many types can be removed by scrubbing with a mild abrasive.

While some of the finishes above can only be applied to specific surfaces, others can be used on a variety of ferrous and non-ferrous materials. Anodizing, while applicable to a variety of metals, is generally referred to these days in relation to aluminum. As with stoving and its related processes, anodizing is available in a rainbow of colors. The greatest weakness of anodized aluminum is mercury, as even a relatively small amount will practically dissolve an object by preventing the formation of a protective oxide layer. Other than that, an anodized finish can be quite wear and corrosion resistant. However, as with bluing, anodizing can scratch fairly easily.

Anodized finish of one of the author's retro ARs

When choosing a finish, or restoring an old or worn finish, consider the application and care requirements. Some of these finishes can be applied by the hobbyist with a moderate investment in materials and equipment, while others are best left to the professional.

Care for finished items is usually nothing more that wiping the object down with a dry, oiled, or solvent-dampened cloth as appropriate. Avoid letting the items remain wet or exposed to chemicals any longer than necessary and they will maintain their integrity for a very long time.

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