This snowy scene with birch trees took just a few hours. The white tree trunks and branches were masked with Pebeo Drawing Gum, which was allowed to dry before I began painting the sky. After pre-wetting the sky area with a clear wash I used Cobalt Blue in sweeping diagonal strokes, leaving the clouds as negative space. While the resulting sky was drying I worked on the snow first with Cerulean Blue then with a bit of Winsor Violet and Neutral Tint. Back to the horizon with Neutral Tint, I shaped the far-away trees, letting them advance and get darker in the woods on the right. The final step was to remove the masking fluid and get to work on the birch trunks and branches, which is where the more fiddly time-consuming work comes in! I try to make these in-class paintings do-able in the time available.
Pebeo Drawing Gum is my preferred masking fluid and the one that I recommend to students. The blue tint keeps the mask clearly visible while you are painting, it stays put until you want to peel it off and then it lifts off readily without damaging the paper.