Nicholas Simons' experiment with soaking strips of dried emulsion glues
was very interesting, matching what I see when I have needed to clean
and repair badly-assembled joints that used PVA glue. A recent example
for me was an unsightly, weak, badly-clamped repair of a diagonally
split, deeply carved mahogany pedal lyre (with faux rosewood graining)
on an 1860 Steinway.
The repair separated easily enough, but took hours of soaking, and
meticulous peeling of remnants of the 1-2 mm thick "glue sandwich" with
tweezers, razor blade, and brass brush before I had enough wood-to-wood
contact in a dry-fit to reglue it properly. I considered using PVA
glue, but used hide glue to minimize the visibility of the joint after
refinishing, and to improve the plight of any future technician who may
need to repair it again. The repair was nearly invisible after shellac
French polish, and the hide-glue joint was strong enough for the lyre
to hold the weight of the piano during moving.
I have only used the brands of PVA glue commonly sold in the US,
and have not used the Evo-stik products common in the UK. I buy
my PVC-E glue from piano supply houses (such as Pianotek
as with commercial products it is sometimes hard to know exactly what
it is made from, and I don't know of any mainstream consumer glue
brands in the US that are PVC-E.
In looking around on the Internet, I found a FAQ page from Bostik that
describes both their Resin-W interior and Resin-W waterproof materials
as PVA-based, presumably with additives in the weatherproof variety to
slow water uptake and/or to stabilize it to UV in sunlight.
I suspect that different rebuilders' experience with getting air-tight
joints between striker pneumatics and the deck in stacks may sometimes
depend on what glue is used to glue the fabric to the leaves of the
pneumatics. In my very first stack many years ago (a Gulbransen) I
used PVC-E to glue the fabric on the recommendation of another
rebuilder, which left a small raised bead of insoluble dried glue at
the perimeter of each pneumatic after trimming the cloth, making it
hard to tightly refit the pneumatic to the deck and get an airtight
joint with hide glue.
It was eventually airtight, but in some cases I had to apply additional
PVC-E glue in the gaps. A space-filling glue like RTV might have been
useful for sealing the gaps better, but at the expense of raising the
pneumatic higher off the deck than its original position, and with the
challenge of adhesive removal in any future rebuild, and with much
longer clamping times because of the cure properties of RTV.
In my second stack, I carefully trimmed the bead
away with scissors or razor blade down to the wood, and found it
considerably easier to get a tight joint to the deck with hide glue,
but it took a lot of extra work, and the trimming into the wood made it
harder to see or 'feel' exactly where to place the pneumatics in their
With my third stack, I finally tried hide glue to attach the cloth to
the leaves, and found that the cloth application was faster, easier,
and much more reliably airtight than with PVC-E. And with normal
trimming of the cloth, any glue bead at the edge was softened by the
fresh hide glue when applied to the deck, so the stack reassembly was
especially fast and easy.
I always check the airtightness of each pneumatic-to-deck joint (such
as by pressurizing the valve opening), but I typically now complete an
entire stack without any that have pinholes or cracks, and in the rare
case there is a small leak, a tiny application of more hide glue
usually seals it entirely.
To my ears, my stacks built entirely with hide glue reproduce music
well, with an example being a small 5'4" Knabe Ampico that I just
finished for a customer this week, which can be viewed on YouTube
or at http://www.youtube.com/watch?v=A1_mUuzUFQY