I use white spirit to clean my paint brushes, as well as to thin my paint for when I am applying the paint in the later layers of the painting. Paint that has been thinned with white spirit creates sharper edges, which is helpful in the foreground areas. This thinned paint will also allow me to add new paint on top of a wet (or slightly tacky) undercoat without disturbing It. For example, when you want to add highlights like grass or foliage to a glaze of darker paint. The one drawback is that by thinning the paint you lose vibrancy density and coverage.