My latest rebuild came back for a tune up. The soundboard, which was very thin to start, got thin enough at the tail end that it began to bulge under string tension.
soundboard very thin here
The first problem was figuring out what to do about it. My first reaction was to consider placing a longitudinal strut between the tail block and the rearward main brace. However, I rejected this because the soundboard had some soft spots, and I didn’t want them to stay that way, fearing future dings might just break the top. So I settled on adding a plate across the tail end that would stiffen the weak zone entirely.
the stiffener
Most of the work was fitting the plate and bracing it. I did a number of dry runs until I was sure it would go in smoothly and could be maneuvered into position. To this end I fashioned some tools to place the plate and brace it.
plate with insertion tool taped in place
insertion tool flips down to brace the plate
Two similar braces were made to prop the wings. Once I got the piece inside I also clamped it lightly from outside to ensure good contact. Because the inside of the soundboard had been shellacked it wasn’t possible to use wood glue, so I had to resort to epoxy. I used quick set clear epoxy, which gave me a few minutes working time, and is also very flexible.
inserting the plate through the soundhole
I inspected the innards in the dark in order to make the new plate. The top is so thin that a weak light will shine right through it in the dark.
the inner light
After a day I strung it up and everything held well. The tone may even be better, a little crisper! There was no unwanted uplift, just the gentle arch as it was supposed to be.
the tail end with proper top curve
nathanguitars 