Hi Maxence,
If you have masked off the fretboard you can go ahead with sanding,unfortunatley its a bit hard for me to give advice on what product you could finish with because i'm not sure what you have available to you there in France. However I'm certain there would be someone here that could give you some better ideas on what to use.
Somebody told me about lemon oil or tru oil, Can It work ?
"Isn't funk the most beautiful creation of man?"
- Volgar, 2018
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#1st build - JMA-1L
#2nd build will certainly be a custom kit close to the GR-1SFG