Well, eventually it came right!  Must have flushed those segments thirty times or more.  The Lacquer thinners did the job, but only when combined with air to blow the dried grease or oil (or whatever it was) out WHILE it was still liquid through the action of the thinners.  Used a very light spray of silicone oil, which worked additional wonders.  I now have 6 CPI available again!

May try that, but here in Australia canned air is madly expensive, IMO.  Going to see how much a small hobby compressor is - have other uses for it anyway.
I know that air pressure (PSI) is something to be careful with - too high can blow springs etc. off their mounts.

Corona Sterling from about 1940 arrived with slightly sticky keys.  OK, been there enough times, so started to flush out the segments as usual.  Things seemed fine, and we were typing well, but then next day they were stuck again, and stuck more firmly than before - and all of them now.  The only result further flushing of the segment has produced is to stick the keys yet more firmly every time, and now it is almost impossible to move them at all unless using force enough likely to break or bend something.
Have flushed with meths (denatured alcohol), turps, and proprietary de-greaser.
Any assistance appreciated.

Mind you, it would be a good way to mark sensitive document copies and make them highly noticeable if they were left laying around on a desk.

I have an SM9 with the same layout; qwertY but with added French and German characters.  I guess it is logical for English language users who wish to include German and French words.  Why not.