Before and after

Madonna, XVIII c. Rome, private collection. Courtesy by 11 [HellHeaven] Art Gallery.

F. Guardi (scuola di), around 1790. Rome, private collection. Courtesy by 11 [HellHeaven] Art Gallery.

Here is a clear example of paint loss due to color flaking:

 

This is the same painting after restoring (oil on copper, Flemish School, XVII c.):

finale