all artist are known to reuse canvases for many different reasons. The most obvious is to save money. Other reasons for painting over another piece are, mistakes on a drawing that can't be corrected, starting a painting and something in the composition just doesn't work or just not liking a finished piece. I'm sure there are other artist that can go on and on about this but those are the most obvious.
DRAWINGS made by Leonardo da Vinci underneath one of his most popular paintings have been revealed. Using a laser technics that are only available now, scientist help The National Gallery in London to discover some things that have been hidden for years.