You have to be able to see what's been done is the basic idea. The U.S. Dept of the Interior guidelines for historic preservation of buildings tell you that any restoration work has be be documentable by future scholars - that's why, for example, you'll see variation in dentil moldings or baseboards in old houses: the idea is *not* to duplicate the older work but to complement it. With the Cimabue crucifix, the restorer did this kind of cross-hatching where the paint was simply gone. When you view it, your eye fills in the details and the piece looks nearly intact, but up close, you can see what's paint and what's restoration.