Comment by crooked-v

It's conceptually similar to current restoration techniques, but, the real innovation is in doing the whole thing as one piece ahead of time rather than requiring somebody to get in there with tiny brushes after a transparent base layer is completed.

It's also worth noting that this doesn't help with the first half of most restorations, which is removing crud and wear and often painstakingly undoing previous alterations or low-quality restorations. There's actually a pretty long history of paintings being altered to fit current fashions or otherwise mucked with in ways that make modern art historians cringe.