You dont say what wood it is...
You might want to grab a can of deft semi gloss, and some aerosol shellac (bullseye).
You can use shallac to coat the headstock front (mask the rest of the neck you dont want shellac'd). Sand it back a little with 320 grit and do that a few times until its smooth. Shellac dries quickly... but like lacquer needs some shrink in tmie - but not nearly as much. Let it shrink in a few days. Sand back the final smooth headstock with some 400 grit, and give it one more shellac spray - light coat is all you need. Then, after a day, apply the decal. Let it dry a day! We dont want water to get into the shellac (any more than we have to). Then give it another light shellac coat. Do not sand. Let it dry a day. One more light shellac coat... do not sand. Let it dry a day. Then one or two coats of the semi gloss lacquer over that.
What you've done is used the shellac as a bit of filler, then a sealer, applied the decal to that, and sealed the top of the decal with shellac (two coats). Then the lacquer ... two coats as a protective clear finish. Lacquer on a decal... has a good chance of making the decal do a resemblance to a dead spider.
The semi gloss Deft is quite matte finish, but no texture. Its flat and dull. The matte Deft is similar but has texture, and you probably dont want that.... so semi gloss is good for this.