Got the fretboard lightly sanded (I wasn't sure how light, so I just gently smoothed it a little with 320 along the grain). Then taped up with electrical tape as per Sonic's suggestion followed by careful application of thin CA using folded squares of printer paper.
Here it is after sanding, tack cloth, and taping:
And after CA application:
The application isn't as patchy as it looks here. As it dried further the appearance evened out a lot. I will let it sit for at least 24 hours before gently sanding back.
For others who might need to use this process as well, here are some more observations:
- The electrical tape worked well. No CA seemed to leak under, and it removed cleanly once the CA was touch dry.
- To apply, I folded strips of printer paper and then applied a bit of CA to the paper. Then the paper is used to apply to the fretboard. The sharp folded edge of the paper was perfect for pushing liquid CA up to the edges of the frets, and for smoothing out the application.
- At first, the paper soaked up most of the CA, but after a few applications it got a good coating and applied more smoothly
- latex gloves are essential. By the end my right hand glove was glued to the CA bottle.
- The electrical tape did lift some small fibres on the maple. It's a good thing I hadn't finished sanding the neck yet.
- Sanding the fretboard trashed the polish on the frets. In hindsight, this is best done before the frets. And given the electrical tape's effect on the maple, this process is probably a good idea right at the start before doing anything else to the neck.