Since most everyone has "painted" in one way or another since they were children, the skill and equipment required to achieve a long-lasting, aesthetically pleasing film are typically underappreciated.
In reality, great skill, discipline, and specific equipment are required to apply a coating on thousands of square feet without visible defects such as brush and roller marks, overspray, lap patterns, shadow-through, dull spots, skips, misses, pinholes, runs and sags, at a thickness that is not too heavy nor too thin. Only a few mils (1000th of an inch) separate these two extremes. And the work has to be accomplished within a wide range of air and surface temperatures, a variety of humidity conditions, and in both still and breezy air. The restrictions on ambient conditions must be met both during application and throughout the drying period.
From Mark Bemus
M. A. Bemus Painting, Inc., Wheat Ridge, Colorado