My understanding is that magnetic flow is what is taking place at the ends of the contacts. Rust does not impede magnetic flow, so clean contacts are not an issue as long as the rust is not so thick to hit the flywheel & create drag. Where the coil mounts can be an issue if it prevents a good ground.
I would want any coatings (paint or WD40) between the coil legs and the magnet.
Maybe that's just me, but my little experience is that anything electrical doesn't like unnecessary resistance.
They make dieletric grease if you absolutely choose to wipe something on the clean metal.
For sure the clear coating would not be legal in tech. WD40 attracts dust and dirt.
If we see a very bad rust problem, we will wipe the part in question with some diluted muriatic acid. Then rinse well with a neutralizing soda, followed by thorough drying and a wipe of gasoline on a rag.
I prefer not to do anything with it if I don't have to. Wipe the rust off with a shop rag and call it a day if it's not too bad.