This magnificent typewriter cleaned up wonderfully well. Most of the nickel bits regained their shine and the whole machine looks very well for a 100 years old.
The decals on the paper-table and on the front-panel were in very good condition, but looked fragile. Especially the paper-table letters appeared to be at risk of just flaking off. To stabilize these, an artist's protective varnish was used - this is sold by art-supply shops to finish (varnish) paintings or watercolours. This is applied from spray can and is promised to be non-yellowing and not attack or make run the underlying artwork - so sounded a good lacquer to protect the machine's artwork.
Before this could be applied, the parts of course had to be cleaned. Especially the paper-table was very dirty - it had dulled from an overall dark covering of decades of dirt. This was very carefully washed away, working around the decals. Using a soft cloth with warn soapy water, the dirt was loosened and then a clean, soft cloth used to take off the water plus dirt.
This protective varnish was first trialed on the paper-table of the test-tinkering machine - to make certain it did not attack the decal or original lacquer. One learning from this try-out was that a wipe with a dusting-cloth right before applying the varnish helps to catch any stray dust-particles.
Also this Underwood had a type-bar rest with a text that marks this as being especially for the Underwood machines with 42 typebars (that'd be a No.5 then).
On this type-bar rest bracket the original lacquer came out from underneath a century of dirt. A deep, glossy black that would have been all over the entire machine - these typewriters must have been a sight when new!
The type-bar rest on this 1920 machine was more tricky to remove than on the 'reference' machine from '28 - so far for its utility as a reference. The 1920 mechanism has an extra lever with boss that protrudes into a slot in the type-bar rest bracket (green oval). Loosening this lever allowed it to shift sideways on its way-rod to wiggle out the type-bar rest.
With the type-bar rest and front panel removed, the typebars were given a polish and the segment slots rinsed with white spirit. This made the 'edge' characters type as light as the 'middle' characters again - the typebars and linkages at the sides (the a, q) are slanted and will have collected more dirt and grime, making them noticeably more sluggish compared to 'middle' typebars (e.g. the g, h or j). (Did not take out the type-bars for a proper cleaning; did not yet feel comfortable loosening the segment - maybe after more tinkering with the 'reference' machine...)
An attempt was also made to adjust the 'L' typebar slug. This was bent such that only the top of the 'l' (or digit '1') printed - making reading typed text a bit difficult. This should have been done with a special tool, but made do with some pliers. Most likely the cause of the mis-alignment was that the slug's solder had come loose and allowed it to shift - and not a bending of the typebar. The slug could be tilted a bit on its typebar. Not wanting to risk re-soldering or brazing the slug, a little cyanoacrylate glue was allowed to seep into any cracks. We'll see how this holds up.
The nickel was polished (Brasso!) and the frame washed. The paper-table and front-panel regained a deep, black shine from their protective varnishing. The frame was given a light rub with petroleum jelly - works against rust and restores a little of its original black lustre.
The rulers' celluloid face was cleaned carefully with damp cloth - any scratches on the celluloid become much less visible after also giving these a light polish with petroleum jelly. Some oil on the carriage way-rod and the whole machine looks most presentable already!
That bell is by the way very tricky to mount with an assembled mechanism. Bell-mounting lug on the frame is again different from the 'reference' machine.
The tabulator rack polished-up magnificently. Unfortunately I made a terrible mistake with the tabulator-ruler. The mounting screws on the '28 machine are NOT exchangeable with the 1920 machine's screws - they are a different thread of different length. Not sensing this quickly enough, I accidentally sheared off the large head of the tiny screw-thread in its hole (a terrible feeling). It is now provisionally fixed in place using some cyanoacrylate, until I can perhaps get a replacement top-bar for the tabular-rack. (Still feel bad about this, I broke it! Despite having a reference machine - actually, because of the 'reference' machine...)
I'll try to forget about that little screw - here looking at the machine from the typing position it does look magnificent :-)