Cosmoline is genericized term for a waxy petroleum product that protects against rust. The term comes from the company named Cosmoline, which makes several different rust preventatives.
Cosmoline is a by product of the petroleum distillation process. It smells and works similarly to petroleum jelly, but has a stiffer, waxier consistency and a greenish brown color with a bit of fluorescence. Cosmoline was originally produced to lubricate, clean and protect military weapons. It was used extensively by the U.S. Armed Forces during World War II. Weapons came packaged in the substance, and it was removed by soldiers in the field. In addition to weapons, it is used to protect outdoor tools, farm implements and marine equipment. Some grades are used to protect and lubricate industrial machines and vehicles.
Because cosmoline is very difficult to remove from surfaces, it has decreased in use as a protectant in contemporary times. Vacuum-sealed polyester film is now the preferred protectant for many of these applications. Old layers of cosmoline become increasingly waxy and crumbly. They need to be scraped off or re-hydrated with an oil before the item can be properly cleaned. On large items, it can be removed with hot water and detergent applied with a pressure washer.