Wow, how did you get the plugs out? It couldn’t be much more than a film of rust. I usually clean old oil and carbon from the threads with a cotton rag wrapped around a dowel sprayed with carb cleaner then turn the dowel/rag into the sparkplug hole.. I’ll follow it up with a dry rag and use anti seize on the plugs. IMO, you won’t get enough rust from the threads harm the engine.
--
A horse walks into a bar and the bartender says, why the long face?