Aleene's Tacky Glue is a proprietary formulation.  They do not list
ingredients on the package.  I have seen it mentioned on material
safety data sheets [MSDS] without the formulation -- they just say
things like proprietary, water-soluble, polymer based.

It does dry flexible, but I've not encountered anyone thinning it,
let alone thinning it enough for the applications I would expect to
see in piano restoration.  It is used in applications where bulk is
desired.

Their sample project directions always use the glue at full strength,
and the object is never just holding things together.  It's put between
layers of paper to allow shaping, used as a coating and burned to
create the impression of aging, used as a surface treatment for its
texture.

Meta Brown
Chicago, Illinois