Just use 2 different products instead (if needed of course): a clear (glossy) laquer (from spraycan) for sealing and a spray adhesive for glueing the damn decal to headstock.
Btw: Jehle's method works fine, but has it's own tricks . First: it was a bitch to get the colour nice and uniform. Second: you'll want to get the decal as thin as possible with no "sharp" edges. Jep, if you use a lot of laquer you'll get it all leveled, but the decal edges migth still be visible. So, for a start sand the decal down "as far as you dare" before glueing to the headstock. After it's done, you can carefully sand even more. I used a lot of slow-cureing superglue (yes, indeed:)) instead of the contact adhesive, pressed it out beneath the decal so that all the decal edges were nicely covered with s.glue (s.glue "melts" the decal material a bit). Beware! you can't just put the decal flat on the headstock, you have to "bend" it on and press all the air out by going. Let it dry overnight. Sanded the edges smooth / removed most of the pressed out glue. What was left formed quite a smooth transition from decal-to-headstock already before clearcoating.
And Jimbo - if you have any specific questions abt. "where to get stuff if you're living on the dark side of the Moon", just check my profile