Artists use turpentine or mineral spirits to thin the paint if they wanted to make a fast-drying sketch that they can then paint over in more detail. The oil paint on the artist's brushes is cleaned out with turpentine after use. Modern chemists have made oil paints that can be used with water. This makes the clean-up job at the end of painting much easier and less smelly. Oil paint is usually dry to the touch in a day to two weeks, depending on how much oil and turpentine is in it. An oil painting is generally varnished when it is finished, which gives a slight shine to the surface and protects it. A painting should dry for several months before it is varnished. An oil painting is not completely dry until it is 60 to 80 years old. Varnishing used to be considered an important part of finishing a painting. Many modern artists do not varnish their pictures at all.