The light changes quickly when painting outdoors. That's pretty obvious, right? It is easy to predict this change when painting a building that aligns with north - south directions. The light hits the East side of the building until 11:00 AM or so. Then there is a transition between 11:00-1:00 PM. This is when I like to take a break from painting for the day. The light then hits the West side of the building. Of course the South facing walls are being lit up for most of the day.
This barn didn't align with those directions. Many buildings do not. The light was the best on this barn for about 45 minutes each morning that I worked on it. Because the barn is weathered, it quickly turned from direct light to grazing light to long shadows from the irregular barn boards, and finally to complete shade. This isn't neccesarily harder to paint, it just has to be planned for. I guess it's these sorts of things that i like the best about painting. There are as many different ways to solve the puzzle as there are painters.