The term "under painting" in the sense I'm using it, is the first stage of a watercolour where broad washes are laid, linking areas which are visually connected, for instance by tone, colour or passages of light and shade. The effect is to unify the painting and to make it flow, by connecting passages which might otherwise appear detached.I use it frequently in streetscapes and the like where light occupies the central core of a single point perspective, flanked on either side by mids and darks . It's also useful in winter scenes or any situation where an all-pervading limited palette of hue and/or tone applies. This is the first of 3 paintings I'm about to post on the subject. They all have common denominators; appropriate scene, a limited sketch (they're all loose paintings; no point doing a tight drawing), major masses of light, shade or colour, and an overlay of rich , finite content which will be laid in the following stage. In this painting of the concourse at Sheffield railway station, I've not shown the first stage, the sketch, but you can see it through the washes which have been laid with a hake. I've graded to the centre light & cut around any preserved lights. Then apply thicker mixtures wet-on-dry or wet-in-wet to suit. The rich, gooey marks of the roof structure capture its presence without dominating the painting by breaking up into dry brush marks. The figure front left is also rendered in rich washes but he is eased in by working wet-in-wet.