Very simple, clean and polish it and then use this to make it like new again. Basically it is black military grade parkerizing. Plenty of YouTube videos on the process.
https://www.lauerweaponry.com/index.cfm?fuseaction=category.display&category_id=1036
You have to do the process in a heated stainless steel tray. Since that is a pistol, you can use one of your stainless cooking pots
Sorry, changed it to the right link...