In addition to being a superb finish for rough service and tough weather, Parkerizing is a good choice for the firearm that has gotten rusty and/or needs refinishing.
Parkerizing has been around since the late 1800's. It has been used primarily as an industrial coating and as an undercoating for primer, paint and other sprayed on or baked on protective coatings. Manganese coatings offer excellent wear resistance when used as a final finish. Zinc coatings offer the best undercoating for sprayed on or baked on coatings. Parkerizing has been the choice of the US Military for years. It is non-reflective, it holds lubricants, oils and rust preventative compounds better than untreated steels or other common finishes. It insulates against electro-chemical corrosion and shields moving metal parts from scoring and scratching. It also inhibits the spread of corrosion.