Paint must be allowed to dry. Some paints such as watercolour, dries in a few minutes. Other paints, such as artist's oil paint, can take days or even weeks to dry. Most other types of paints fit between these two. Many types of paint are best if they are put on in several layers. The painter must first prepare the surface so it is smooth, clean and dry. The first layer of paint is often an "undercoat" or "ground" which is smooth and sticks very well. The coloured paints are put on over the top. A house-painter who is using oil paint on a wooden window frame might used three layers of paint to give very good protection from the weather. Other types of modern house paint only need one layer, particularly on the inside. Artists using oil paint and tempera often use many layers to get just the right effect.