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Varnish is meant to be stripped every 40-60 years.

For a stark example of how conservation effects the appearance of a work take a look at this image of the Mona Lisa, and the Pardos Mona Lisa replica (copied ~20 years from the original). Because the replica is much less valued, more aggressive conservation and restoration techniques have been applied. Compared to the original work which last saw major conservation over 200 years ago.

[1] http://www.arthistorynews.com/i/entries/1069.jpg

I never knew there were other copies of Mona Lisa from Leonardo's time period, so to save others some confusion when googling: OP is talking about a copy of Mona Lisa in Museo del Prado in Madrid.