Pretty much what Uwe said, but also adding some details about my experience with electrolysis. I have used electrolysis to remove rust from the panels of an old Underwood SS, among some other large pieces of metal, personally, I would not recommend it if it is your first time with electrolysis or don't have the proper tools or implements. Here some considerations:

* The paint will be ruined, so be ready to do a full paint job. This by itself is a problem if you want to respect the original aesthetic of the machine according to it's age. The old black shinny ones and the wrinkled ones are the hardest, specially the later; the late 50's, and later plain paint jobs are easier to replicate/restore. 

* There will be a nasty goo all over the metal. Nothing a brush and some cleaning agent can't fix, but it will be messy.  

* There will be no paint and no filth. The bare metal will be as clean as it never has been, let's consider it is a complex mechanism (think in the segment and all the inner levers that make the machine work. Though the idea sounds fantastic, it also is a problem that leads to the old, and well known, question of "should I oil my typewriter?" You see, as clean it could be, it is also VERY vulnerable to rust and it will rust, fast. Personally I would use a really light lubricant for the sake of protection to the inner mechanisms, but consider this also might lead to the problem of catching dust and eventually jamming the machine. If you know the right oil or the proper way to avoid a quick rusting and/or the accumulation of dust, go ahead.  

* Is not only steel and iron. Nickel, copper and tin can also be found in the standard Underwood (at least in my experience). Consider that the nickel plating can be damaged and the chemical byproducts of the electrolysis can be a hazardous by themselves. This also brings back the matter of the paint, have you considered it's chemical composition?, the paint of back then is pretty much hazardous compared to our present standards.

Though I can't deny I am curious about a "full machine electrolysis", personally I would try to disassemble it as possible (I think "The Classic Typewriter Page" has most of the documentation you might need in regards disassembly). Perhaps not to the level of separate every single piece but to the point of have one block of mechanisms, while removing the parts that are in good condition, don't need electrolysis, or the rust they might have can be cleaned in a different way (a wire brush or naval jelly comes to mind, and some more information about rust removal...).

I might be wrong, but In regards standard Underwood typewriters (in case we are talking of an standard typewriter), I  think the SX 150, and later models, are easier to disassemble, specially in separating the outer case/shell from the inner mechanism block. Earlier models not so much. I have no experience with the portable Underwoods. 

Tell us how it goes, also some pictures could be useful, best of luck (:
 

Thanks; I'll be redoing it without whole-body electrolysis. It's pretty nasty. So I will be dismantling what I can of it & then using Evapo-Rust as has been mentioned. 

The Underwood is an old No. 5 desktop and it is missing a mainspring barrel assembly & a pivot for the tab rack (plus feet...and I likely shall redo that platen while I am in there.) It is incredibly rusty. I don't know what else is missing. The serial number I had thought was a 1917 number. It is instead from the late 1920s. I do not remember off the top of my head what year it is exactly. But anyway, it is still an Underwood. 

Oddly enough the shift lock is on the right-hand side. 

The machine is pretty sorry looking but it's yet there. If I had an image hosting site I'd put a picture up for you. 

Thanks for all the helps! 

Does anyone know when Underwood switched to black keys with white lettering? The paper inserts are black instead of white.

Hey guys, the repairs aren't quick but I'm digging it apart. This one will run again if I can find some pieces.

Thanks much for all the info about rust removers.  I'm going to go with something like Evapo-Rust. Turns out a lot of pieces are nickel plated on this machine and so it looks like I shall be building a plating bath as well, eventually. 

The paint is going to have to come off. It has holes in it and looks beyond shabby. What strips that black asphalt paint well? 

curly wrote:

Any update on the progress for this? I'd like to see some pictures if you have them. I have an old Remington 11 desktop that will need a complete teardown. There is a Facebook group for typewriter parts.

Hello! Glad to see the old Forum isn't dead. Sure, I'm actually working on it. 

The motor came from eBay at $9 and I just recently joined Facebook to get into the typewriter groups. You can buy 'em, sell 'em, trade 'em and show 'em off there. Hm. I am 21 and thinking about getting into typewriter repair as a business or at least a side hustle. 

Disassembly went well. The Underwood comes apart in a series of assemblies. I don't know how the Remington 11 works; I have a Remington Paragon 12 and it's similar but...it has side plates & I don't take them off to just stare at the guts. 

 If I could upload a file I would.

Anyway, the Underwood has had a number of challenges so far. One is organization. I have had to put every screw back where it came from and that helped a lot. Two is rust. Salt air since the 1940s had tore this one up pretty bad. So I have taken an old greasy piece of sandpaper & scrubbed rust off the space bar, only to find the original blued finish underneath. OK, color me surprised.
I then went on to disassemble the machine to basically the frame & key levers & springs all in one assembly, and then the carriage way rod, and the ribbon winder assembly & its shaft. The segment came out but all the typebars are apart.
more than likely I will end up redoing the nickel plate on them, and every silver piece of this machine.

The platen shattered like a vase when I pulled the rubber off & struck it on the workbench. It needed replacing. I'm going to do a heat-shrink polyolefin rubber platen. Keys are going to be Ames printouts to replace the originals, might as well while I've got the rings off for plating and also so I can clean under the glass.

Feet will be No. 4 test tube stoppers.

This whole thing is a big mess! I think the paint is going to be ground off or stripped somehow and then I am going to repaint and then decal the whole thing again. I expect it to be a 3 week job for painting alone. Reassembly is going to be the easy part with all the surface prep.

Anyway, it's happening & I just wish I could share pictures in this forum. Let me figure out a way to link some Instagram pictures in here & that will work.

You guys don't know how much teenage me used to stare & look at this forum, just admiring the typewriters & listening to the restoration war stories. Y'all are the G.O.A.T. Keep being great!