Protect your Grand Piano with a: 100% Wool String Cover
A Funny Story (Caution - not recommended)
I recently went to a customer's house to tune her piano. For some reason the string cover had been removed and left out of the piano for about a year. When I opened up the piano I found that it was quite dusty. While I removed the action to do some regulation work, she said she would get the dust out of the piano. The two pictures below show how she did it.
Customer opens up the windows, plugs in her leaf blower, and Z-O-O-O-M, out the window goes all the dust.
While the blower did a fairly good job of getting the dust out, I do not recommend leaving the string cover off the piano for any extended period of time.