I’ve come into a little bench-top jointer of somewhat advanced age. It’s similar to the one pictured here. I’ve got a motor that will operate it, once I find a belt, and the knives seem to be sharp, and can probably be made more so. But the poor little tool was left outside for a period of time. I’ve started cleaning it up, but the bed and fence need a little more than WD-40 and elbow grease. There are some scratches I’d like to polish out and generally ensure that the surface is as smooth as possible so I don’t damage the wood when I run it across. I’m looking for advice on how to do this - what types of tools, abrasives, and/or chemical agents will help restore these surfaces to their original mirror-like smoothness quickly and easily? And hopefully cheaply… 
Well, I’d use very coarse sandpaper. But the issue is not just removing dings, but keeping the bed flat. Anyway, it doesn’t need a mirror polish. Pitting isn’t going to cause any problems. You just don’t want anything sticking up that would scratch the wood. I guess I would try 60- grit sandpaper, ideally self-adhesive, with a dead-flat lapping plate, and use a good straightedge to make sure you’re keeping the thing flat.
Scotch brite abrasive pads do an excellent job of removing corrosion from a metal surface.
Better still is a thing sold as a “rust eraser.” But my experience is that these things are good for polishing up a surface, not for removing lots of metal and flattening. If you have nibs or bumps that need to be ground down, you have to use a grinder, or a flat plate with coarse sandpaper.
I’d suggest taking it to a machine shop which can plane the surface and make sure you get something flat. Also get new set screws to hold the blades, and replace any shims that have become corroded. Make sure your motor is matched to the tool HP wise, and your drive pulleys give you the proper operating speed. Joiners may be the most dangerous woodworking tool out there.
Once you get the steel back to whatever condition you want, Boeshield works nicely to protect it from rust. Yes, that funny spelling is from Boeing, and its original purpose was to protect airplane parts. It’s not a goopy storage protectant like Cosmoline - this stuff can be applied to machinery in active use.
I’ve got some swords that turn brown if you just say the word “humid” near them, but Boeshield has kept them rust-free and fingerprint-free. It’s also great on cast iron surfaces such as table saws. One of its several claims to fame is that when applied properly, it will not transfer to the wood you’re running through the machine.
You can find the stuff at places like Rockler and Woodcraft.
Thanks for the suggestions. I’ll probably end up using a few of them in combination as I’m facing a little of each of the problems you mention.
I’m especially interested by Boeshield, which I hadn’t heard of before. I had some reloading dies get very rusty last winter, so maybe I’ll be able to multi-purpose some if I can find it.