Here I show the varnish between two frets on a varnished maple fingerboard. 

This fingerboard was sanded and varnished. Only the result was not satisfactory. Here I show the repair, where it also looks good afterwards.

For this operation, I have small radius sanding blocks for the corresponding fingerboard radius. Some 180'er sandpaper stuck on the bottom and carefully sanded off the existing varnish layer. You can also see in the picture above that the radius was off due to the previous sanding. So sand a little more into the wood.

Round the edges of the fingerboard again, then mask the neck and varnish with satin clear varnish. Depending on the color of the wood, it may also be necessary to tint the first coats a bit to make the difference between old and new disappear.

You can only see it when you know it!