Sign painting is the act of taking a specific kind of brush, and with various kinds of paint, and applying it to a 2 or 3 dimensional surfaces creating letters, forms and/or symbols.
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. (text via Wikipedia).