One of the most critical links in tone for your guitar is the bridge. So many customers come in for a check-up and don’t realize their bridge is lifting slightly from the top. Sometimes it is very clear with a big gap behind the bridge, but early on in the development. It can be a minor line that you won’t see unless you look! This is a very common guitar repair issue.
One of the wrong ways of dealing with this is to shoot glue into the gap and clamp it back down.
Unfortunately, it is a common error made by repairmen. The wood of the soundboard and the bridge still have the old glue in there and it won’t make a good bond unless the bridge is carefully removed and the old glue scraped away to bare wood on both surfaces. The bridge is then glued back on. The tone will jump when this is done correctly, often surprising the owner, who has become accustomed to the decreased quality of sound from the loose or lifting bridge.
The photo here is my vacuum clamping system, which is amazing. No inner block or “clamping caul” is needed as there are no mechanical clamps. Simple negative air pressure brings the top and the bridge together at their natural memories, and the clamping pressure and glue “squeeze out” is perfectly even across the entire gluing surface. No compression “hot spots” exist and the vibration form your strings will travel across the bridge evenly and cleanly. It is also safer as no metal parts or clamps are used that can dent or scratch your finish.