I used to prime MFD board to carry along, but since I've been prepping Thunderbolt cardstock (Kraft ph neutral 80# paper), I have found it more convenient both to paint, and then thereafter to store (either in a plastic bin, paintings separated by wax coated paper...or a small art portfolio). There are just a couple examples...of practical application.
Setup on the Temperance River...along the west shores of Lake Superior, in Cook County...and painting in the Grand Marais Plein Air event...I was looking at the mouth, the light, visitors (often skipping stones once they got out there), and realized that morning, that the light was definitely going to change in a hurry, and change the whole sense of the drama in the small gorge. I decided that a couple quick 4"x 6" taped off prepped cardstock paintings would be ideal...to decide if I would like the horizon of the distant water high with less sky, or low with more sky...and get the shadows down?
It proved to be a good plan...after choosing the second quick study, because as I believed it would...the light having moved into the gorge created an entirely different setting. The subject remained worthy as a reference, but it no longer held true to the compelling drama that I wanted.
My paint students proved to be fairly new at painting outdoors on location, so I demonstrated a simple palette, first a values driven black and white mix of a dark, mid, light plus white...and then did the same with color choosing several dominant colors of the scene to set up my mixing palette with a dark, mid, light of each color. If the work required more color transitions, I would leave the option over to include two halftones to each main value group.