Over the last few days I have been getting a lot done. Almost 100% of the rust is gone after soaking it for a few days, and in the meantime I've continued with my restoration. First up, I've started taking a small screwdriver and removing the spring bits that are still stuck inside the keycaps. I got a batch of about 30 done yesterday and put them threw about 20 minutes of ultrasonic cleaning (my cleaner actually died, so hopefully it still works after letting it sit for awhile). They are looking much better and the water turned a gross yellowish-orange color. I have also started working on the hammers and have been removing any remaining spring bits from those as well. I've currently got about half of them done and I'll be putting the new springs on soon.
The PCB is looking really good considering how it did a few days ago. There is a bit of damage around the sides of it, so hopefully it doesn't affect the circuits at all. All I did was soak it in my solution and everything is looking good as new. The backplate had a weird grayish patina on it that was a bit sticky. I have no idea what it was, but I sanded it down and tried to remove as much of it as I could. The plastic separator also has that weird gray gunk on it, so I may make a new one if necessary instead of trying to remove it all. I lightly sanded the barrel plate and sprayed the antique nickel Rustoleum on it alongside the backplate. It looks really good in my opinion, I'm not sure if it will even need more than one coat. The derusting formula managed to remove the staining from the case and it looks great now!
Almost everything is starting to come together at this point and I am starting to see the light at the end of the tunnel so to speak. My only hope is that the keyboard actually works after all of the work that I've put into it.
I was able to get it fully disassembled yesterday and was initially a bit taken aback by the amount of rust present. However, it actually cleaned off very well. I washed everything with some water and antibacterial kitchen spray which removed probably 80% of the rust and dirt from everything. The PCB is still stuck to the backplate since the two screws holding them together were too rusty to remove. I removed all of the hammers and barrels and placed them to the side. Everything is now taking a bath in some rust removal solution (which is completely safe for use on as plastic).
As for the metal components, I'll be lightly sanding them and then spraying them with antique nickel Rustoleum in order to prevent any further corrosion. The barrel plate is actually immaculate other than a couple of light scratches, but I'll be painting it too since there is some bare metal exposed. Hopefully I'll be able to get the remaining screws out once I pull them out of the bath so that I can finish cleaning everything completely. The internal springs for the feet on the case must be in pretty good condition despite the rest of the keyboard since the feet work perfect and there's no spring noise.
I also have the barrels, keycaps, and hammers soaking to remove as much of the rust and discoloration as possible. The plan is to remove the remaining bits of springs still stuck to the hammer stems, and then replace all of then with good condition Model M springs. The foam is in really good condition, but I might consider replacing it while it's already open. All in all, I'll now about $30 into this project after buying the rust removal solution, a plastic bin to soak everything in, and spray paint. I think there is a very good chance of getting it all working again so long as I replace the controller.
Got it taken apart today, and only broke one tab! I think I can glue it back on though, as it is not completely broken off. The screws holding the assembly down were rusted worse than anything I've ever seen and even broke a screwdriver when trying to get them out, so I had to get my ratchet set and wrench them out instead. The plate is in surprisingly good condition considering the rest of the board, and I think that a light sanding and painting should fix it right up. The case actually has a lot of character, so I'll probably give it a light cleaning to get the loose grime off and then sanitize it and leave it as is. I might see if there are any industrial shops nearby with an ultrasonic cleaner, but I'm not sure it's worth the time and effort to get to a place. Even more surprising is that the foam is in outstanding condition from what I can tell. The next step will be to get the assembly taken apart and (hopefully) salvaging the old hammers so that I can replace the springs. This will be quite a mess so I'll probably be doing this outdoors or tape a bunch of plastic to my floors and do it there. As an added bonus, there is a perfectly preserved and rusty paperclip from 30 years ago in the debris if anyone wants it!