I don't believe there is a problem today concerning the deterioration of Leonardo da Vinci's religious masterpiece, The Last Supper (1498). The painting underwent a lengthy 21 year restoration between 1978-1999--one of many through the centuries--and it is still housed in its original location in the Santa Maria delle Grazie in Milan, Italy.
Since it had proved impractical to move the painting to a more controlled environment, the refectory was instead converted to a sealed, climate controlled environment, which meant bricking up the windows. Then, detailed study was undertaken to determine the painting's original form, using scientific tests (especially infrared reflectoscopy and microscopic core-samples), and original cartoons preserved in the Royal Library at Windsor Castle. Some areas were deemed unrestorable. These were re-painted with watercolor in subdued colors intended to indicate they were not original work, while not being too distracting.
The two decades-long project was meant to correct the damage done by poor restoration projects of the 17th and 19th centuries as well as the dirt and pollution of more recent times. The refectory was damaged by World War II bombs, though the painting was not hurt. Yet another restoration of the work was undertaken during the 1950s.
Da Vinci originally painted The Last Supper on a dry wall of the monastery, and then sealed the stone wall with "pitch, gesso and mastic" before adding the tempura. The painting began deteriorating a few short years after its completion. It began flaking less than 60 years after its finish, and was virtually "unrecognizable" by 1652. A curtain was hung over it for protection during the 18th century, but instead of preserving it, the curtain
... instead trapped moisture on the surface, and whenever the curtain was pulled back, it scratched the flaking paint.
Viewers of the painting must make reservations ahead of time and are only allowed a maximum of 15 minutes inside the refectory. The last restoration was criticized because of the great differences in "colours, tones, and even some facial shapes."