Piano Technician's Perspective
Why 100% wool felt?
Unlike other fabrics, 100% wool felt has unique moisture-moderating properties. The exterior of the wool fiber is hydrophobic while the interior is hygroscopic. It absorbs a certain amount of excess moisture into its interior, and also helps quickly evaporate the rest away. While other fabrics might make effective dust covers, only wool offers this type of moisture moderation.
For instance a good sailor's sweater is made of 100% wool because if the sailor gets splashed, his sweater will get dry again quickly.
Note also that your own hair is similar to wool, and if you get your (non-woolen) clothes and your hair wet at the same time, your hair dries much more quickly than the clothes.
Wool felt string covers are well field-tested. I have seen 100% wool felt string cover installations going back to the 1950’s in Miami Beach Florida, (which is a humidity nightmare locale for pianos). The amount of corrosion seen in such pianos was a small fraction of what is normally found in pianos in the same region that did not have string covers.
As another example, I once tested wool felt and non-wool fabric strips by soaking them and hanging them up side-by-side to dry. The wool felt dried in a fraction of the time the other fabrics dried.
Wool felt is already used as under string felt and damper felt in pianos. When de-stringing a piano with rusty strings, note that wherever the felt touched the strings, that segment of the string is usually rust-free.
And think about how wooly unshorn sheep are. If wool did not help moderate moisture, a sheep that had been exposed to prolonged rains would likely develop serious skin problems related to mold, rot, fungus, etc.
Thanks to all you piano technicians/tuners who sent these perspectives to us!