Whatever feelings engulfed Titian or Leonardo, if they suddenly found themselves in one of the great art galleries today? Perhaps they initially would not know some of their works. And when, after tactful questioning, would admit them, first they would have been shocked, then outraged. They could probably understand the need to move their operations from the churches and palaces in art galleries. Sensible people, they would not be surprised that their paintings have suffered, going from hand to hand. They have a philosophical attitude to the tracks of time on the canvas and colorful layer, and with the darkening of varnish, they have encountered. But they would be horrified at how their works have suffered from human interference.
It was noted that there are two correct ways to ruin the picture: to restore it or restore. This dilemma is often justified. Strangely, there is scarcely any public debate about the restoration of paintings. The fact that almost no audible differences, should have alerted us. Emerging discussions impermanent, indecisive, and very few give information.
http://ec-dejavu.net/r/Restoration.html