Firearm coatings have become very popular recently, especially using the Cerakote technology.. In fact so popular that some manufacturers offer it as an option, like Glock and FN. The point of mentioning it here is to suggest it to those whose needs require a higher level of protection, like those who carry daily, those who live in harsh environments, and certain military applications.
NRA's Shooting Illustrated did an article on Cerakote in December of 2016 and had some interesting comments:
A picture of a Glock 19 with a Cerakote finish (https://www.cerakote.com/gallery/?htids=1505)Developed by NIC Industries, Cerakote is a thin-film ceramic coating that offers a hard finish that is resistant to abrasion, corrosion and chemicals—and looks great at the same time. The thinness (0.001-inch thick) of the coating makes it ideally suited to the often-tight tolerances of firearms. It can be applied to practically all materials used in firearm construction, whether it's steel, aluminum or plastics/polymers. While NIC Industries does not do any application work itself, you can check its website to find a local Cerakote-certified applicator.
Another important consideration during the development of Cerakote was its ability to bond to materials other than just carbon steel. With the growth of the use of polymers in the 1980s as well as the established use of aluminum, a coating that could adhere equally well to carbon steel, stainless steel, aluminum and polymers was an absolute must.