Luthier Tips du Jour Mailbag - Crack Repair

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In this episode, Robbie O'Brien addresses a question from a viewer in Brazil about crack repair. ‍

Video Transcription

[on-screen text reads: Luthier Tips du Jour Mailbag]

Mailbag question: My guitar has cracks in it. Can you show me what to do?

Robert O'Brien: Today's Luthier Tips du Jour mailbag question comes from Brazil. "Robert, contrary to popular belief, there are some places in Brazil that are very dry. My guitar now has cracks in it. Can you show me what to do? Thanks for your videos, Eduardo."

Well, Eduardo, we've been talking about this for the last few weeks. The first thing we've talked about that you're going to notice with changes in relative humidity is action problems. The top is going to swell up or shrink and it's going to raise and lower the strings and cause you action problems. The second thing you're going to notice if it progresses further is fret issues. And we addressed that with Stefano in Italy. The fret ends were sticking out. I showed you how to take care of that. And the third and final stage is usually cracks. These cracks can be cosmetic or they can be structural. In the majority of the cases, they're usually just cosmetic. However, if they progress further, they can become structural and this can be a problem for you. So Eduardo, let me show you what to do if you develop cracks in your instrument.

So here's a guitar that came into the shop with some cracks in it. You'll notice here there's some cracks right along there that need to be addressed. In fact, they have been addressed in the past. However, this particular musician goes out on tour quite a bit and the guitar is subject to extremes and changes in humidity, and the guitar is cracked again. They've already been addressed structurally. Now, how would you address it structurally? From the inside, you would place small cleats generally of the same species of wood along the crack there to help stabilize it. This has already been done, so all I'm going to show you how to do is the cosmetic work. And the cosmetic work involves a couple of stages. First of all, repair the crack. Second of all, repair the finish.

Repair the cracks

The first thing I would do after addressing the structural issues, which this has already been addressed, is to cleats on the inside, is come in and fill the gaps. Now if the gaps are large enough, you can come in with a dark piece of veneer like this, Brazilian rosewood, Brazilian rosewood, place it inside the crack and then flood it with a little bit of CA glue, clip it off, scrape it, sand it, you're good to go. These cracks are not that wide enough, so we don't need this.

What I'm going to do is come in with some rosewood dust, work it into the cracks, and then apply the CA glue. So with the sawdust inside the crack, I then come in with a little bit of thin viscosity CA glue and just place it in there like this. That will also help stabilize the crack and the sawdust is helping to level it and fill the void. And if you want to place a little bit more over the top of that you can. Come in and rub it into the crack. And I have been known to also take a piece of sandpaper and lightly hit the top of the crack as well.

Notice that I did not use accelerator on here because the accelerator has a tendency to turn the CA glue white. Now, if you need to flush things up, if they're not quite level when you get done, here's the little trick I learned. I believe it's from Frank Ford that I learned this one from Just place some pieces of tape around a razor blade. The clear cellophane tape allows you to scrape it right down, flush with the finish [Robert works the explosed razor blade over the filled crack lines]. There we go. Now everything's level and ready for finish work.

Repair the finish

Fixing the cracks is sometimes the easy part. Now I've got to deal with the finish repair. In this case, the finish is shellac.

[Robert rubs the shellac into the repaired crack areas using a circular motion with a rag.]

Well, Eduardo, thank you for your question and I hope you have found this information useful. Remember, humidify your guitars before it progresses to this point. So Eduardo in Brazil [foreign language 00:03:56].

[on-screen text reads: More Luthier Tips and online courses available at]



Robbie O'Brien

Luthier and Instructor, Lutherie Academy