I just did an underpainting today. This was all on the advice of a fellow painter here, who says he does a thin color underpainting using the oil paint thinned to the consistency of ink or watercolor.
Progression is below:
Above I started with a line drawing to block-in the layout using a small round synthetic brush, and a paper towel corner dipped in turp for an eraser.
Above I used the burnt umber thinned with turp to lay in the basic lights and darks.
For the underpainting I used turp-thinned paint and kept the values light and the colors reddish. Everything is just a tint.
This is the underpainting as it stands so far. My plan is to continue working on it for a couple more sessions and see how far I can develop the painting in on-site. You might recognise this as the same boulder I drew back on Day 2.
Yesterday evening we went out to do another sunset study. This one again suffers from being too light. Not sure I'm cut out for the lightening-speed approach this requires to capture the hues, values and chroma.
Here's a shot of several of my fellow painters finishing up their studies just after the sun has set. That's Hunter Mountain beyond us, you can see the cut trees from the winter ski trails.