To Bruce Clark's excellent diagnosis of Ampico Leaks, I must add the
glued-in block at the ends of each deck.  There is a block of wood the
exact size and shape of the air channel down the center of each deck.
This is glued in and shellacked over at the factory.  You will notice
that the seam line around this block is usually black, indicating
leakage and sucking in dirt for years.  Using the stethoscope you can
hear the large amount of leakage coming through these loose glue joints.

We sand them down to wood and always cover the ends of the decks with
motor cloth or pneumatic cloth.  This prevents a future problem and
cures the problem if it is already happening.  If the stack is already
in the piano and you don't plan to take it out, sanding off the shellac
is not absolutely necessary if you use PVC glue.

D.L. Bullock
www.pianoworld.us