Back Crack Repair - Luthier Tips du Jour Mailbag
In this episode, luthier Robbie O’Brien shows how to repair a crack along the center seam of a guitar back.
Video Transcription
[on-screen text reads: Luthier Tips du Jour Mailbag]
Mailbag question: I made a mistake and glued in the reinforcement strip for the back in the same direction as the back grain. Is this going to be a problem?
Robert O'Brien: Today's Tips du Jour Mailbag question comes to us from Minnesota. "Robert, I am following your online course and I'm to the point where I glue in the reinforcement strip on the inside of the back. I made a mistake and glued it in the same direction as the back grain instead of perpendicular to the back grain. Is this going to be a problem. Phil, in Minnesota."
Why you should install the center reinforcement strip perpendicular to the back grain
Phil, if you were using the LMI materials, this is what comes in the box of materials from LMI. It's a center reinforcement strip to go on the inside of the back and all of them are made to go with the grain perpendicular to the back grain. And that's to help keep it from separating as it expands and contracts. Now, I know some very big name builders, world-renowned builders, that put that center reinforcement strip going parallel to the back grain. I prefer to put it going across the grain. The reason why is because I just had a guitar come in. It's on my bench right now. It came in yesterday from Granada. It's a fairly new guitar. It came to Colorado. Colorado's climate is dry. The guitar has dried out and the back has separated. If I look inside the guitar in the soundhole, that center reinforcement strip that goes parallel to the grain has split. Even the label that is glued over the top of that has cracked and split down the middle of the label. So the technical term for that is bad.
So if I were you, I would take that off and put the center reinforcement strip perpendicular to the back grain.
How to repair a guitar with a cracked back
Now, the problem with the guitar on the bench, I have to fix it. So what am I going to do? There's a couple things you can do. One is to humidify the guitar, put some glue in and hope that that guitar doesn't dry out again and separate. And I'm not going to rely on the owner to take good enough care of that guitar that it won't separate again. So in my opinion, I need to fill the void somehow. So let's step over the bench and I'll show you how I'm going to repair this guitar.
Seperate the purfling lines
So here's the guitar I was telling you about, Phil. It's got a very nice binding and purfling line down the center of the guitar. It's got a very thick piece of rosewood. And then on either side of that, it's got a light, dark and then light. And what has happened is that the light purfling lines have separated and there's a space in there where that dark line is and it is so large that I can put a razor knife in there. I'm going to run my Razor Knife down in there and separate, or unglue, that brown purfling line from the white one. It was glued on one side to the white line, and then on the other side, it stayed on this side and they came back to this side. So I'm going to just separate it and push it all over to one side, like so. So I'm pushing that dark purfling line over to one side.
Fill the void
The next thing I did is I took a piece of veneer and I took my hand plane and a razor blade and I made it into the shape of a V or a wedge. And the idea is to fill that void now by just pushing that down into the void. And obviously, I'm going to have a little bit of glue on there. I'll let it dry, then I'll hand plane that down, scrape it down and then I'm looking at just the cosmetic repair, which is the finish. So I'm going to place a little bit of glue into that void there [Robert works the glue into the void with this finger]. Then I am going to come in with my veneer and place it into the void. There we go. So wet paper towel will help clean up the excess glue. There we go.
Hand plane flush and polish
Now the next thing to do is let the glue dry, then come in and hand plane that down or scrape that down flush with the back. Now that the glue is dried, I'm going to come in and take this purfling I stuck in there down flush with the back, and you can use the chisel to do that. You can also use a hand plane and finally, the scraper. Once the repair has been done from the outside, now it's time to work on the cosmetic aspect, which is the French polish. So after some quality time spent doing the French polish, the repair is now complete and ready to go back to the owner.
So Phil in Minnesota, thank you very much for your question and I hope you found that information useful. Now, if that guitar had stayed in Granada, perhaps that issue would've never have happened. But since it came to an extremely dry climate and the guitar was not humidified, that's what happened. So keep that in mind when deciding which way to orient the grain on that center reinforcement strip, and happy building.
[on-screen text reads: More Luthier Tips and online courses available at www.obrienguitars.com. Private and small group guitar building and finishing instruction available.]
