The cleaning of a painting should only be undertaken by a trained paintings conservator. Most paintings involve two layers of cleaning. The first removing layers of dirt that build up naturally. The second, mostly on older paintings, is the removal of discoloured varnish. Over time these transparent varnish layers can become more opaque and discoloured resulting in the original paint layers being viewed through a distorting yellow or even brown filter that can alter the original colours and tones of a painting and flatten the three dimensional effect.