Christ by Jan van Hemessen (the sixteenth century) was too provocative. It was painted dark and the clothes were added. Today, we can see the original again.
An art restorer is like a neurosurgeon that has no margin for error. If you choose the wrong solvent to remove old varnish, you can accidentally remove age-old paint from the canvas. Restoration can’t be rushed and you have to be really careful while doing it. This is why it may take 6 months to restore just 1 inch of a painting.