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Collings Mandolin
Nasty Top Damage
© Frank Ford 2005; Photos by FF
The bad news: This nearly new Collings mandolin had suffered a really nasty bit of top damage when a novice human (a.k.a. "toddler") grabbed it by the most fragile part of the f-hole and ripped out a nice big section of the top.
The almost good news: The piece broke out very cleanly with no extra splinters or finish damage.
My job was clear enough, and I set about protecting the inside of the instrument with a paper towel that would catch any drips of glue or finish.
I daubed on a thick layer of fresh hot hide glue. Hide glue was an obvious choice for this job because it sets up to a very rigid joint, has good strength, and is the least affected by heat. So, when the repair begins to age, the glue is unlikely to shrink, swell or stretch which might translate through the finish as raised or sunken glue lines.
A bunch of tiny C-clamps provided good vertical alignment for the crack, and, instead of making a sophisticated curved caul for the top surface, I used some thin celluloid guitar picks, each of which bent nicely to conform to the concave arch under each clamp.
Once the glue had dried and the clamps were removed I prepared a bit of heavy cotton canvas reinforcement material. It seemed an obvious choice, considering the compound curves in that area.
I soaked the canvas in hot hide glue until it was thoroughly saturated. Once in place, the hide glue would form a rigid matrix with the canvas weave and the unfinished spruce inside the top.
Well, at least there's one advantage of having skinny fingers. I was able to reach inside the f-hole and smooth the canvas neatly in place. After the patch dried, I gave the f-hole "wing" a little flex test and verified that it was a good deal stronger and stiffer than the undamaged one on the bass side.
Laying a piece of fine sandpaper in an analogous area with the grit facing up I was able to contour the bottom side of a tiny cork sanding block.
Here, you may be able to see that the sanding block exactly fits the curve.
That little curved block made it possible for me to "scuff" the repaired area and level the edges of the cracks while maintaining the original contour.
As my first step in the finish touchup process, I applied a stripe of slow acting solvent (actually lacquer "retarder" thinner) to the nitrocellulose lacquer finish. The solvent melted the flaky finish at the edges of the repaired crack and adhered them to the surface of the wood.
Next, a quick brush of new lacquer. Collings uses a very thin (somewhere close to 0.003") lacquer finish, and I wanted a little extra thickness there to allow for my scraping, probing, sanding, or whatever.
Using brown stain dissolved in MEK, I dabbed on new coloring over the bare wood that showed through at the cracked areas. Since I had already sealed the cracks with lacquer, the stain didn't bleed under the finish, or soak into the wood, so I was better able to control the color.
But I still got a fair bit of the colorant on top of the original colored finish, making for dark outlines in the repaired area. Here's a hot tip: after the stain dried, I smeared a layer of lemon oil over the finish to act as a "lens."
Then, as I scraped the dark color away from the raised edges of the crack leaving the stain in the recesses, the oily surface continued to look just as it would under new lacquer. Without that oily lens, I'd have had a difficult time judging the color in the area where I was scraping.
Fast forward a couple of weeks, and a couple of new coats of clear lacquer, leveling and buffing. A solid repair that's still noticeable if you look for it, but it's not too obvious.

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