Sign painting was/is a learned craft. It has a very long history within the realm of "artisans-crafts". Historically, apprenticeships were the means of learning the craft. Though many, in the earlier history of the craft were self-taught. An apprenticeship could last for years, depending on the skill of the apprentice and the knowledge of the "master". The skills learned were varied and some quite complex. Basically, learning to manipulate a lettering brush was the core of the learning process. This skill alone could take years to master. There were a number of associated skills and techniques also taught, such as: gold leafing (surface and glass), carving (in various mediums), glue-glass chipping, stencilling, silk-screening.
With the advent of the computer and various kinds of software now available the sign painting craft has been displaced with computer driven sign making machines. The "craft" has all but disappeared, and in only a few "technical schools" or specialty schools is the craft still taught. Such as L.A. Trade Technical College.
Sign painters are usually self-taught and/or taught by mentors in the business. This is because Sign Painting is rarely offered in schools/universities, which in turn is the reason it could considered a dying trade. However, most professional Sign Painters are quite passionate about their work: computer-generated signs are both a blessing and a curse.
See also 
- Article on an exhibition of historical roadside signs in New England
- The Letterheads Website - The Keepers of our Craft - Sign Painting is alive and well!
- The Original Letterheads - A site maintained by one of the founders of the Letterhead movement
- Creative Signmakers of America - Internet Forum for Creative Signmaking