Hey Bruce. Thank you! I did it! I first de-stringed, then straightened the neck by adjusting the truss rod. It was almost there anyway after the strings came off. (I used a straight edge down the middle, illuminated from behind). Then I started full length strokes with the long sanding block. Immediately the string grooves and buzz points (I had already mapped) were highlighted as shiny. From there it was simply a matter of working the whole neck until all the low points were almost completely gone, and then I finished off. There is still plenty of meat on ebony, so this could be done again (but not likely in my time). It is now playing like a new neck and has plenty of whaa where it should. A deep crack is now much diminished and no longer causes buzz.
I have a Maton (Australian) from about 1986. It had one previous owner who thrashed it.
I have found your tips from this and other threads extremely helpful. I now have a beautifully sounding and easy to play instrument.