I don't remember if it was here or on another list but some time back
I wrote about the experiences I've had when covering organs and pianos.

The admonition I'd pass along to the next person who wants to cover
their instrument is do not use any porous material in direct contact
with the finish as it will absorb moisture from the air and hold it
against the finish.  This can totally destroy some finishes, as I can
attest to, having had this happen to two organs that I covered with
"piano covers" directly over them.  Add to them, a third instrument
that was covered with a very nice quilted blanket.

In one instance, the case on a Hammond X-77 organ was literally
destroyed from the moisture.  Even the wooden blocks at the sides
warped so bad they had to be replaced.

Now, I use small plastic "tarps" and have satisfactory results with
them.  Even in winter, moisture can be a problem in some places.

Hal Davis