When you view the video you will see 2 interesting procedures that are indirectly connected.
The first is the painting of the canvas with a flood of colour diluted with white spirits. This is allowed to stand as long as possible for the white spirits to evaporate. But sometimes the process has to be helped with a hair dryer and the excess wiped with a dry tissue. Because different coloured paint is applied relative to the final content of the painting, this is called underpainting by some painters, as opposed to staining of the canvas. Underpainting usually refers to layers built up, before the final skin is applied as in the multi-stage painting process of letting the initial paint layers dry before proceeding with the next. ‘Alla prima’ (as this painting is), or one session, wet on wet painting can’t really have an underpainting, because its all wet, and mixes together into a single homogenous layer. So its not, strictly speaking, underpainting.
The purpose of this layer of dilute paint is to modify the subsequent colours applied. As there was going to be a lot of white colour applied, for the snow, the paint underneath interacted with the white and produced a range of hues and tones impossible to produce by mixing on the palette and applying individually. Another incidental advantage is that the final paint can be applied without trying to completely cover the canvas. If there are ‘gaps’, the under layer eliminates the stark white of the blank canvas. This brings us to the second procedure – scratching or scraping the wet paint to reveal whats underneath. As mentioned in the previous post, painting the fine lines of the trees, on the left, into the thick wet layer of sky colour was going to cause problems. By scratching the fine lines and filling them with the dark colour of the trees was fine for the thicker branches, but some scratch marks were not painted into. These were OK as the under-layer of the dark colour was uncovered. There were also a few scratches made here and there to help integrate the ‘scratch’ texture across the entire surface.
Hopefully the following video will explain the process a little better.