Well my case is somewhat special... I had to "attach" the fiberglass paste to the neck, but in fact I think I'll not do that anymore. I've found there exists another kind of paste, in spanish "mastique" that looks much harder and resistant to humidity.
My concern is that I'll probably need to sand after applying it, and I don't have a curved sanding brick (or however it is named in english... you can now say confidently that I'm not a native speaker hehe). I guess I'll have to look for one of those at the end... the thing is that if I end up buying one of those, I would probably be better off doing it right and using wood veneer