Thanks @dm7 :)

The drawing is made on wood, and tend to be be far more resilient than on paper, so no use of the usual graphite/pastel fixative .

It needs to be protected though, and that's the job I gave to acrylic blender (can be seen at 30s or so in the video).
When dried, you can use brutal tools like glasspaper : graphite won't get away easily (useful to get to get a damaged/worn out effect)
Just be sure it's dry before starting to work over it though :)

Hope it'll help :)