My Secret for getting quality sound out of Disk Piezo pickups.

[G.S. Monroe]

This help an awful lot with reducing feedback when you are installing disk piezos in your box.

Use it to shield your exposed disks, and solder connections. I make a point of using it with all my wiring.

[Wichita Sam]

G.S.- Good for you.  Any PLIABLE coating (such as silicine caulk) that covers the back of the disc, cutting off ambiant vibrations, greatly reduces feedback.  I've experimented with hard coatings like wood putty and hot glue but they didn't heelp as much.