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A really bad one
Fingerboard Top Crack
© Frank Ford 2005; Photos by FF

It's a D-45, Martin's traditional "top-of-the-line" guitar, and from a distance it looks pretty good, particularly without the tension of being tuned to pitch.
Close up, though, there's mighty serious trouble. The top has cracked right alongside the fingerboard, and the sections of the top under the fingerboard has slid forward to the point of virtual collapse in that area.
A tension, you can see that the neck block has slipped inward as well, pulling binding and purfling loose in the process.
The soundhole rings have buckled to an extent that doesn't even look possible without breaking.
The treble side looks even worse.
And, you can see how the crack follows the purfling that's cut halfway through the top in this vulnerable structurally important area.
All this movement would not have been possible without the glue failure associated with exposure to high heat, such as that found in a parked car. At around 150 degrees Fahrenheit, the aliphatic resin glue used to construct this instrument can fail almost completely.
I reached inside with my little palette knife to see how loose the top bracing was.
You can see in this mirror reflection that the braces were totally loose from the top almost all the way across.
The first order of business was to remove the large flat braces on either side of the truss rod, and the little flat ones near the soundhole. I just pried them off with the palette knife.
Once those braces were out, I put a small pad under the neck block, and clamped the end block to the edge of my workbench. Then, I applied clamping pressure at the peghead end of the neck until the top sections were drawn into correct alignment.
With the guitar clamped this way, I could go about reinforcing the area to avoid future slippage.
First, I reglued the large tall brace above the soundhole and clamped it securely after working some glue into the cracks alongside the fingerboard. After the glue dried there was enough support to allow me to remove the guitar from its clamped position so I could continue the job.
Because there were lots of little splinters still adhering in the areas where I removed those braces I turned to "RoboSander" for a bit of help. It's a little tool I made up for sanding inside. I have four of those strong rare earth magnets mounted on the end of a shaft completely enclosed in this housing I made of PVC pipe and a thin piece of acrylic.
The shaft is supported by a heavy sealed ball bearing
Inside the guitar I placed a matching quartet of magnets glued to a sheet of coarse abrasive.
As I drove the outer magnets with my drill, the inner ones would spin to level the surface quickly and neatly. Since the outer housing doesn't rotate at all, there's no chance of scratching the finish.
I replaced the upper flat braces with the widest ones I could make to span the space between the neck block and the upper brace.
To avoid the slippage that had occurred with heat exposure, I used hide glue for all these operations. Hide glue is the least likely of any instrument glue to be damaged in heat, and it never becomes flexible the way aliphatic resin glue can.
And, I made a wide flat brace to go between the brace and the soundhole.
It's so much easier to clamp in an oversize brace than it is to make one that fits exactly, I didn't trim it at all until after the glue was dried.
A big spruce piece like this is easily split away.
I trimmed the patch back under at about 45 degrees so it wouldn't be easily visible from the outside.
This is my replacement flat brace to go at the treble side of the soundhole. I made thicker and notched it to lap over my new wider upper flat brace.
More hide glue, more clamping. I made a matching brace for the bass side of the soundhole, too.
Here's a mirror view of the rebuilt area.
And, a closeup of the overlapping brace. If you look closely you'll see that I added a corner block to each side of the neck block to carry the load across the crack in the top - just a bit of extra "insurance."
After all that distortion, I was amazed that the plastic purfling was not broken anywhere, and only one piece of abalone inlay had been lost. I made a new mitered piece and inlaid it.
A bit of brush finish touchup was all that I needed to add over the damaged areas. Finish adhesion was good on all the wood surfaces, but the lacquer had chipped badly off the abalone and plastic.
Some delicate scraping to level my new finish addition. Here, I used a glass microscope slide with tape protecting the sharp edges over the original finish. Using this technique I was able to scrape my new finish down to almost the same level as the surrounding lacquer without scratching.
Some fine sanding (1200 grit) and buffing completed the job.

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