The carrying case of the Corona Four typewriter was still sound and relatively undamaged. It had of course accumulated 91 years worth of dust, dirt, scuffs and corrosion. A few simple steps can make the case much more presentable again and help prepare it for its next 91 years.
The first thing I do is to clean it with a very little water and soap. Some lukewarm water with a strong degreasing detergent is good. I'll use washing up liquid or general household cleaner, the strong liquid stuff. (These shouldn't contain any alcohols, many hand-soaps do!) As a first quick action, wipe all surfaces gently with a damp cloth (not wet!) to get off the superficial dirt. Then there'll still be lots of dirt however.
With a soft (!) toothbrush some soapy water is brushed onto the dirty surface. Making small rotary motions the dirt is worked loose, then a rag or paper towel is used to pat it dry and take off the dirt. In this picture of the case lid the left side has been cleaned, the right-hand side still shows the dirt of decades clinging to the case.
When doing this, take care not to use too much water; that's bad for the wooden case and may cause the leathercloth to warp and lift. Also don't rub hard or use a hard brush as that might well scratch the surface.
The inside of the case is treated the same way - especially the case bottom was very dirty and needed several goings-over. The bit in the back has been mostly cleaned, the rectangular area in the front-left only wiped off - there is a difference :)
After the case has been cleaned and left to dry (though it should not have gotten really wet from this), small repairs are made. That is mostly glueing down lifted edges of the leathercloth and sealing the edges of scuffs with PVA glue (known trans-Atlantically as 'Elmer's Glue' I believe). As always and everywhere with gluing, it's best to use very sparing amounts.
When all the loose bits are firmly attached again, the case is given a polish. It is 'Leatherette' of course and not leather, but regular black shoe-polish works well. It adds some extra black to even out the colour and the wax does help to protect and soften a bit the dried out leathercloth. Apply with a cloth (press in when needed) and polish up to a bit of a shine with a soft (!) shoe-polishing brush. The case lining was green, so that's polished with some natural wax (furniture) or Carnauba. (For coloured cases, e.g. a gramophone case, I'll sometimes shop for a shoe polish of the right colour.)
Before doing anything else, it's probably a good plan to let the case stand for a few days. It'll have a strong odour of concentrated shoe-shop for a few days. With any remaining solvents evaporating it also gives the wax some time to form a proper protective layer. (The whole polishing thing is best done outdoors anyways, with all the solvents in a polish...)
Then the metal case fittings - these can be said to have 'patina', but in this Corona Four case I think it's just dirt and rust.
With the power of Brasso metal-polish and a cotton rag, that case lock can look much cleaner again. Polishing the metal fittings by the way I'll do only after giving the case a first protective polish. Any spills or rubbing of metal-polish can then be wiped off clean again and won't stain in the leathercloth. (When fittings are just too far gone and only pitted, dark metal remains, I'll paint the fittings black. A gloss or satin smooth finish generally looks fine and definitely is cleaner.)
When polishing the inside fittings to bring back some of their shine, a sheet of thick paper or cardboard is held next to them to protect the case lining next to the fitting.
With some more Brasso even the plain metal handle latches look better again.
Again fit for 'home-use' instead of being something that needs to be 'kept in the shed' :)