This late 1972 Guild twelve string came to me in need of some repairs. My initial inspection of the guitar showed that the bridge was well seated around its perimeter but the 'belly' of the bridge was separated about 1/16" from the top and that void was filled with wood glue. I wouldn't let that ride on a six string and it is definitely not good with the extra tension of this guitar. That will never do.
I used my vinyl sealing iron to warm the bridge and applied De Glue Goo, a gel vinegar solution, to the glue to assist in breaking the bond. The process took about thirty minutes from start to finish. Once the bridge was removed, I realized that this wasn't simply a lifting bridge but rather a full bridge reglue that went awry. I wasn't the first person in here.
We are looking at pieces of top wood missing and pockets of glue in their place. Neither of which are signs of a good reglue. Wood glue is a poor gap filler as its meant to bond two, well-fit, surfaces together. Further, wood glue bonds poorly to itself so applying more glue on top of the already dried glue is a waste of time. This needs to be fully redone if the I want this guitar to have any change of survival.
Surface Prep
With two, freshly-sharpened chisels I began carefully carving away the glue and excessively damaged spruce. I make up a couple of small sanding blocks using scraps of wood with bevelled edges and 120 grit sandpaper glued to their face. These blocks are used to level the surface and prepare the spruce for the patches that I will be gluing in. I've also found it helpful to apply low-tack masking tape to the top, trace the bridge outline, lightly score the tape, and use that to clearly define where I need to stop.
I have removed about 1/32" of wood, about the most I'm comfortable with, which has exposed fresh, clean wood.Notice how towards the perimeter of the bridge I have removed less spruce than the center, that is crucial to the success of this repair.
The strength in this joint is the bond between the long side of the patch (shown in yellow) and the long side of the recess in the top. In a quarter-sawn board, like the spruce top and patch, these sides are known as the 'face' grain. Wood glue excels at bonding timber on the face or side grain but is weak bonding 'end' grain. We have to consider this when inlaying the patch.
In red, you see an example of weak glue joint where the end grain of the top is glued to the end grain of the patch and both boards meet at a right angle. I'm not going to claim any formal knowledge of engineering, but my understanding is that right angles 'concentrate' stress and will break first. My own experiences in woodworking and repair support that understanding. A repair using this technique will certainly fail.
In green, you see an example of a strong glue joint where the clear line between end grain and face grain is fuzzy, there is a mixture of both. In addition, the taper allows both materials to flex as the patch is less rigid where it is thinner. This spreads the stresses out across both pieces.
Making the Patch
Quartersawn spruce stock is an odd thing for a hobbyist to find but I've found that its easiest to find a wood or lutherie supplier and buy a B stock top, an orphaned top, or a spruce bridge plate. It pays off for making cleats and patches. I suppose one could find quartersawn spruce veneer but I haven't found it necessary. Using the right species and cut of wood makes the repair look professional and spruce has one of the highest weight to strength ratios of timber. It makes sense to keep it consistent.
First, I use a thickness (or drum) sander to reduce the spruce to about 1/32". Usually these patches will sit proud and have to be pared down. Then I mark my desired area on the bridge, align the spruce on the bridge, and trace the perimeter. I use a bandsaw and spindle sander to shape the patch. Lastly, I sand a matching bevel into the bottom of the patch and then, on the top, sand a wider bevel to allow the patch to flex while I clamp it.
The patch sits in its channel and resists being moved around, this is exactly the fit I was looking for.
I made up a couple cauls out of some scrap maple and tapered them along their length so they have a little flexibility. My patches don't sit perfectly flush with the top, at this stage, so the flexibility is beneficial. I use a plastic straw with the end cut at an angle to scoop up the excess glue and I usually apply masking tape to the bottom of my cauls to prevent them from being glued to the instrument.
Finishing
After the patches have had a chance to dry, I come back in with chisels and working in a slide-to-slide, slicing motion to carefully trim the spruce back. This would be considerably easier with a cranked-neck chisel but I don't own one yet. The sanding blocks come back, again, to level the surface. Masking tape is very important to prevent damaging the lacquer.
As I work, the edges of the patch become translucent and you can see the original wood underneath. I'm not trying to glue the bridge onto a platform but, rather, bring the low spots up to meet the original wood. Near the top of the photo you can see one of my patches doesn't extend the full length since the original wood was in good condition there.
I check my work using a ruler and find that the surface is consistent, level, and at the perfect depth for the bridge. The patch can't be much thicker, now, than 1/64" but it has a clean bond to the original wood and will cleanly bond to the bridge.
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