From Engineering Design Handbook Ammunition series section 6:
Protective Coatings. Two types of protective coatings are used on steel cartridge cases, zinc, or phenolic varnish. Zinc is prescribed by the Navy and varnish by the Army. The cases are prepared by removing scale, formed during the mouth anneal, as well as shop dirt and any traces of machining oil. The varnish coat is uniformly applied by dipping. After dipping, the coat is air dried prior to baking, in order to avoid thin or bare spots resulting from flow of the varnish on first contact with the heat of the oven. The phenol formaldehyde resin varnish specified for this service bakes by polymerization at a temperature of 400F. The varnish case is tested by pouring 3 liters of Ottawa sand upon the inclined sidewall through a vertical tube, 3 feet long, located 1 inch from the case. The area abraded must not exceed a specified limit. The baking at 400F also serves as a lowtemperature stress relief wich protects against rupture during firing, but does not appreciably lower physical properties. Cases wich are to be zinc plated are stress-relief annealed after the taper but before plating. Before zinc coating, the case is cleaned electrolytically in an alkaline bath. After rinsing, and an acid dip to neutralise all traces of the alkali cleaner, the outside and inside of the case are plated to a thickness of 0.00015 to 0.00020 inch.
I have a 75mm RCL case wich was rusty cleaned and oiled with lijnolie (forgive me I only know the Dutch name) and used a component for drying. It looks nice enough for me.