Good question, here is what to do. Carefully remove the screw holding the rubber grips, set them aside after wiping the back of them of any old oil. Get some CLP Breakfree from Walmart or such. Get the type that comes in an aresol can. Spray liberally with the gun in a small tupperware or ziplock bag. Use the red hose attachment to spray the inside under where the grips are, down where the hammer recess is when cocked, etc. That usually will suffice. Let the CLP soak on the outside too, heavy. Why CLP Breakfree? Because it is made to Clean, Lube, Protect, and because it's thin enough to soak into places like a penetrating oil, but not so thin it won't lubricate, like Kroil. It has excellent corrosion prevention in normal home environments. "Saltwater spray testing" you see on the internet is not what you are going to be experiencing with a nice vintage gun unless everything goes wrong. Use CLP or Kroil.
Do not try to take the gun apart unless you know what you are doing. Without a properly fitted screwdriver, you will mar up the screws, reducing value.
After it soaks and drains, wipe the outside with a cotton cloth. Turn the cloth often to use a clean, white area - the brown rust particle are abrasive, and will scratch the good blued parts. Keep the rusty parts soaking in oil for a couple days, repeating the process. That will stabilize the red rust and stop it. Do not use any steel or bronze wool or abrasives. That is an advanced technique for an expert. It will NOT hurt it to leave the rusty spots for years if you keep oil on them, after oiling. It WILL hurt the gun if you try to agressively scrape off the rusty spots, leaving the pits they caused behind.