To remove the rust that is already on the plunge posts, I would recommend using very fine uncoated steel wool. You might want to wipe the posts down with a mild solvent like denatured or 90% isopropyl alcohol. I would recommend steel wool instead of non woven abrasives, since the non woven Scotch Brite type abrasives have aluminum oxide embedded in the pads than can come loose and get embedded in the bronze bushings that slide over the plunge posts. The steel wool idn't much harder than the plunge posts and at most will burnish any roughness on the surface making the posts a bit smoother. This might not remove all the staining from the rust but the posts will wind up fully functional.
After using the steel wool it's probably best to wipe the posts down with alcohol again ti remove any dust left from the steel wool.
Then as other members have already recommended, I would use Boeshield T9 on the posts. The Boeshield goes on as a tin liquid than can get into any pores in the steel of the post sealing them.
If you have it, after using the Boeshield, I would recommend wiping and polishing the plunge posts with Renaissance Wax. The Renaissance wax is formulated and used to protect objects like armour at the British Museum, and also leaves a very smooth hard surface coating. The wax finish is harder and smoother than the finish from the Boeshield, and the plunge posts should plunge easier and more smoothly one applied. I know after applying the renaissance wax to the plunge posts on a couple Dewalt routers, I could plunge the routers by a single handle, which didn't really work previously. Regular paste wax might also workbut I don't think it dries as hard.
