While that may work for you, I wouldn't recommend using a sharp object on the fingerboards.
Easiest and safest way to get the grim off ANY fingerboard.....use an old toothbrush!
What I use...
-Foam Windex (staples or any place should sell it)
-Planet waves Hydrate or Boiled Linseed Oil (Rosewood/Ebony Boards ONLY)
It's simple and effective. (and it won't harm the wood).
-Spray Foamy Windex on fingerboard ( do about 7 frets at a time)
-Use Old Toothbrush,"brushing" both length and width of board.
-Wipe off dirt with paper towel.
Once this is done, let dry for a minute or two. (or if you have an air compressor you can blow it out.)
*If you have a Maple Fingerboard your done.*
**I would normally steel wool the board (Rosewood/Ebony) now, but I would skip this step as it could leave scratches on the board if your not careful**
For Rosewood or Ebony boards only.....
-Using Planet Waves Hydrate (We Sell it..
) or Linseed oil, apply to board. Not too much, but enough to cover the entire board. Wipe on with paper towel. Let it sit to soak in for a minute, wipe off excess with paper towel.
-Let dry, restring .