Here's a couple of oil sketches I did.
(Sketch for Sunset Bluff. 22x20cm oil on board. © Andy Dolphin)
(Sketch for Behind the Bluff. 20x22cm oil on board. © Andy Dolphin)
These are small sketches, done mostly with a single brush so there's not much in the way of fine detail. The major struggles were keeping track of the changing colours and trying to lock in the shadows that tend to stretch rapidly as the sun sets.