In the event i needed to plate a complex part such as an oil line, i would look at doing the plating myself, so i could have control over the process. In past decades, i seriously looked into home plating process/ingredients/kits. As i recall, zinc plating is a straight forward process. There are kits available to buy or at least there used to be. Maybe the environmentalists have killed that fun. Just need to know equipment, ingredients and process. The only thing that could be a bugaboo would be the issue of having to have the aluminum parts with the steel parts in the electrolytic solution. Have read one can make a simple enough fixture to hold the portion of a part one whats to plate in the electorlytic solution. If i were to attempt plating oil line ferrous parts, i would research process, build the setup, and use a "non-sandcast" oil line as the guinea pig for testing the plating process to see if it actually works or until i got the process right.