If your not an expierienced luthier, mabey you should give it back and tell him to take it to one. I don't want to sound like an ***hole, but its better its done right than having the neck come undone while the strings are tuned and tense. Unless it a bolt-on, then its cake.

As far as the paint, the clearcoat finish isn't a good surface for any paint to cling to, so you could probably just use some warm water and a rag, or use a plastic razor blade to scrape away the paint, leaving the body un-scratched.