Bluing is used to improve the cosmetic appearance of, and provide a measure of corrosion resistance to, their firearms. Bluing also helps to maintain the metal finish by resisting tangential scratching, and also helps to reduce glare to the eyes of the shooter when looking down the barrel of the gun. All blued parts still need to be properly oiled to prevent rust. Bluing, being a chemical conversion coating, is not as robust against wear and corrosion resistance as plated coatings, and is typically no thicker than 2.5 micrometers. For this reason, it is considered not to add any appreciable thickness to precisely-machined gun parts.