The ruler on older portable typewriters is generally a stamped bit of metal with embossed markings. The markings are usually coloured in with a contrast colour, white lettering on a black ruler. Now this lettering today is often faded, dirty, rusty or partially flaked off.
How to revive or re-apply the contrast colour is very helpfully explained in the book "Algemeines Anleitungsbuch Für Schreibmaschinenreparateure". A general manual for typewriter-repairers is exactly what we need here! Though the book is oddly low on pictures, it is very rich in useful advice.
Must have been a very useful book back then in '26 and it is so again almost a century later. Many thanks to Georg Sommeregger for scanning and sharing this!
On page 59 the improving of rulers is explained. A combination of zinc-white and wax is recommended, heating the ruler a bit and then rubbing it on. A mix of zinc-white and wax, now that would be a white wax-crayon. Heating the ruler a bit (~70° C) with the crayon, this indeed rubs nicely in the embossed markings. The excess wipes off clean, leaving the wax solid in the markings as it cools down
The ruler of the Remington portable typewriter being refurbished was unreadably rusty, so was totally stripped clean and repainted. Now also with new bright white lettering from white wax-crayon.
Found that there's a difference in quality of the wax crayons; the 'cheap' items don't contain much (or any) wax. They're probably purely synthetic. These don't work at all for this. Proper wax crayons however work well and soften and melt as you'd expect wax to do. (Used Caran d'Ache brand wax-crayon on this ruler.)