Here are some excerpts from a publication that is sent to all FFLs.
From http://www.atf.treas.gov/pub/fire-explo_pub/qa.pdf (Questions and Answers)
I. GUNSMITHS
(I1) Is a license needed to engage in the business of engraving, customizing,refinishing or repairing firearms?
Yes. A person conducting such activities as a business is considered to
be a gunsmith within the definition of a dealer.
[27 CFR 178.11]
From http://www.atf.treas.gov/pub/fire-explo_pub/part178.pdf (Commerce in Firearms and Ammunition)
27 CFR 178.11 Meaning of Terms
Dealer. Any person engaged in the business of selling firearms at
wholesale or retail; any person engaged in the business of repairing
firearms or of making or fitting special barrels, stocks, or trigger
mechanisms to firearms; or any person who is a pawnbroker. The term shall include any person who engages in such business or occupation on a part-time basis.