Piano Hammer felt filing
A piano technician came to tune and regulate my upright piano that I moved from my parents' house. Over time and use, the hammers wore out and needed to be filed...
(Piano hammers are the single most significant factor that influences the feel and tone of pianos.)
With the cover removed, the piano seems very naked.
I can see some of my mechanically-minded students would be all over this. They would love this uncovered secret.