For a Maple board, Fender fit the nut first, then spray the finish (no masking tape).
Then the top of the nut is (usually rather roughly) sanded to remove the finish.
There is always some finish on the front and back of the nut, as well as on the sides.
For a Rosewood board, they mask the board and the front of the nut.
Gibson also do the same, masking the board and front of the nut.
Most other manufacturers (PRS, Taylor...) fit the nut after the neck is finished.
As a result, removing a nut on a PRS takes about 3 seconds.
Removing a nut on a Fender or Gibson can take up to 20 minutes!
The finish first, fit nut later method is kind to techs.
The fit first, finish after looks "authentic" (vintage?)