This anonymous cityscape subject lends itself well to this technique. Like the other two in the genre (Sheffield Station and Westbourne Arcade) it's an out-take from one of the three watercolour classes I run, dealing with a specific subject. Once again the important thing is to spend a little time studying the subject before laying paint, being clear in your mind as to what the main characters are - shapes, colours, values - where the light must be preserved in the wet-in-wet washes, and in particular where to cut around and leave white shapes to come back to in the later stages - in this case the reflections on cars and the tram's windscreen. Your analysis will tell you which marks should belong in which stage; under-painting or over-painting and frequently the stages might overlap, developing and building continuously in various areas at various times, because the timing is right. The whole process is based on ever-richer mixtures as the painting develops, much like the fat-over-lean method in oils but it is important that you understand the relationship between pigment content and time lapse, dropping onto areas when appropriate. This is watercolour's biggest issue and the student's most difficult lesson to learn. You should only paint wet on dry when you choose to, not because you're a victim of poor timing.