Thanks, Dan and Dave - Several technicians have contacted me privately with questions about how I set up the wire and half-round clamps. Here's the answer I sent them, hopefully it will make the procedure a bit more clear:
Hi xxxxx - It's not terribly complicated. First, glue (I like Titebond, or Bouldoc's acoustic wood glue) is worked into areas of separation (between the ribs and the soundboard) with a narrow piece of veneer. Wipe off the excess.
Thank you for that document , I will have a look ater.
To insert glue, what was used is the metal piece used in women underclothing before WWII (in metal, but I heard some where done in Whale teeth .
I was given an excellent tip lately : as the space to pour glue is so thin sometime, if you have an old metal rule , the kind that is rewind in a box, you can dismount it and the spring inside is avery thin and resistant blade that can pour glue in thin places.
the pressure is then installed from the most open side to the opposite, so tu push the glue.
I dont know what method you use, I made rib clamps with a curved shape, a little long, with 3 tuning pins for instance, possibly even longer clamps could be used.
In soundboards there is a part of the elasticity that is due to the glue joint between ribs and panel, and between bridge and panel.
This is the part that connot be restored, in my opinion, may be the use of a first grade hide glue can help adding a little stress (good hide glues are incredible in the way they tense the material they glue) I was lucky enough(§) to reglue all ribs on a small old grand (totally unglued) and had a really differnt result from the usual rib reglueing (with more of the "recent piano" tone (hence more dynamics)the soundboard gained some boom and resiliency certainly.
WHat a friend explained me is that because the wood age, the panel get smaller, so when he repairs old fortepianas, he add some wood to the panel to correct that.
If the whole panel is alittle smaller probably only a complete regluing of the ribs (and the bridge ?) can reinstall enough resiliency.
That does not mean an old panel is getting a little more life when repaired, but I never find them accepting more downbering after the repair, if only shims and local rib gluing is done.
The DW seem to be better but the panel lower where it was precedently after some time. Adding downbearing only for a few years is probably less good than doing nothing.
Raising the board curve certainly is the way to go, but lowering the plate, no in my opinion.