The original Ektachrome slide dates back 30 years and has suffered only slight fading and staining. Under normal conditions, this would be a very straightforward restoration job. Unfortunately, the slide suffered water damage at some point, and it has developed a very serious case of mildew. More than half the slide has some mildew damage; about one-third of it has very severe mildew damage. In some areas the original image is barely visible through the filaments. The damage is deep enough in some small portions near the center left side that the original image has been entirely eaten away by the mildew.
Restoring the original tone and color of the slide was easy; there was only modest loss due to fading and staining. Undoing the mildew damage consumed 90% of the restoration time. Spatial filtering and brush tools could repair about half the damage, but there were myriad obscuring filaments as fine as a few pixels in width in the heavily-detailed cityscape that required pixel-by-pixel repair.
A hugely difficult job, to be sure. But physical restoration would have been impossible. Even cleaning the original would be dangerous, because of the very fragile nature of mildew-digested emulsion.
(Time to restore: approximately 30 hours.)