20th Fret Treatment
A common mistake is cutting the 20th fret slot all the way across the fingerboard. If you do this you will end up with a tiny slot on the bass side. You can add a tiny fret there and it looks ok. I used to do it. But its not the norm. The problem is that there is so little wood after the fret that most of the time it will break off. It can be a pain to repair since the fret will just keep pushing it out. I got tired of doing this and realized I was probably the only one putting a tiny fret there.
To avoid this you should only cut the 20th fret slot half way across the board. I usually stop just before the entire blade disappears under the fingerboard. But fear not, if you have been over zealous and cut all the way through there is a good fix that is undetectable if you do it right.
Filling in the slot with stick shellac, ebony dust and glue, or replacing just the tip of the board up to the 20th fret will be noticeable. Seams across the grain are impossible to eliminate. Seam along the grain with most dark woods are undetectable is done right.
To fix the problem you need to rout out the bass side of the fingerboard from the tip of the board to the 19th fret slot. The drawing will make it easy to see what you need to do. The notch needs to be at least as deep as the slot and just a bit wider than the tiny slot you are trying to remove. This can be easily done using a laminate trimming router with a straight bit hand held. You then only need to clean up and straighten the inside edge of the notch with a chisel. Its not a scary as it sounds.
Now cut a piece a bit wider and higher than the notch from the fingerboard cut-off. If you use the piece off the bottom of the board you can get an almost exact grain match. Square up the piece so it fits up against the inner edge of the notch perfectly. Make sure the piece hangs over the side and the bottom of the board. Glue in place and trim when it dries. If you covered a section of the 19th fret you can then remove it with a dozuki saw. My dozuki has a .026″ wide blade which is perfect for cutting the slot. And viola! You are done.