Soundboard crack- glue(s) and repair humidity, Ding

Construction and repair of Classical Guitar and related instruments

Soundboard crack- glue(s) and repair humidity, Ding

Postby StevSmar » Sun Feb 12, 2012 6:04 pm

The Classical Guitar which I made (In Australia at 70%+ humidity) developed two cracks in the Cedar soundboard a few years ago (Winnipeg winter at less than 30% humidity). The cracks are 3-4 inches long and on either side of the fretboard, where the neck joins the body. There's also a nice ding courtesy of a friend dropping the case lid on the soundboard.

My questions are:
1) What glue(s) to use to repair the crack.
2) Whether I should wait until the humidity in our house is above 40% before repairing the crack.
3) How to repair the ding in the soundboard.
4) Should I varnish the inside of the guitar to slow down how quickly it dries out.

For the cracks, I plan on gluing cross grain Cedar patches on the underside of the soundboard with Aliphatic Resin (Titebond). One of the cracks is very tight, and I doubt whether I can get any glue to penetrate into the crack. My thought is to use superglue from the outside, once the patches are in place. Other posts have suggested superglue risks staining the wood, but that's not really a concern for me. Alternatively I can thin some of the Titebond and paint it on the crack from both the underside and outside before I glue on the patches.

For the soundboard ding, I have previously steamed out dings in balsa wood model aircraft by spraying the ding with Windex, and if necessary putting a bit of heat from an iron on the ding. I will likely also put a small patch in the area of the ding.

I'm not planning on re-finishing the top once I make the repairs, especially since my feeling is that my guitar is going to continue to develop cracks since there's such a difference between the humidity of where it was made and where I live now.

Thanks for your suggestions.

Stevens Guitar.jpg

Two cracks next to fretboard.jpg

Soundboard ding.jpg
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Regards,

Steven from Winnipeg
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Re: Soundboard crack- glue(s) and repair humidity, Ding

Postby Marcus Dominelli » Sun Feb 12, 2012 8:49 pm

StevSmar wrote:My questions are:
1) What glue(s) to use to repair the crack.
2) Whether I should wait until the humidity in our house is above 40% before repairing the crack.
3) How to repair the ding in the soundboard.
4) Should I varnish the inside of the guitar to slow down how quickly it dries out.


Hi Steven,
1) Hide glue works great for this. It's strong, penetrates cracks well, and does not irreversible damage. Don't use crazy glue. It can discolour the wood, and can potentially create other problems.
2)If the cracks are closed (no big gap) then go ahead and glue them, and cleat the inside of the crack after it's dry. The hide glue should help swell the wood and close the gap if the gap is not too large.
3)The soundboard ding is a bit trickier, and I don't have a quick answer on the best way to do that - sorry.
4) It won't slow moisture exchange down much at all, and the finish will just complicate future regluings, or cleating, should these cracks ever re-open and need to be fixed again.

Dominelli Guitars
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Re: Soundboard crack- glue(s) and repair humidity, Ding

Postby Alan Carruth » Sun Feb 12, 2012 8:58 pm

The quicker you get to steaming out a ding, the more successful you're likely to be. After a while they sort of 'get used to' being dinged. I've had good luck with putting on some water, allowing it to penetrate, and using a hair dryer or heat gun (carefully!) to warm it up. This will get out surprisingly deep dings, so long as the wood fibers have not been damaged. That one probably can't be made to 'go away', but you might get it more level.
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Re: Soundboard crack- glue(s) and repair humidity, Ding

Postby Vesavekkuli » Tue Feb 14, 2012 9:38 pm

Here in North-Finland the winters are very dry - humidity 20-25%. The only way to manage with fragile wooden instruments is to use an effective humidifier in the room - if that is not achievable, use a special humidifier inside the guitar. I wouldn't wait for more damage to appear without doing these arrangements. About the ding: I've used succesfully many new layers of finish - sanding between and carefully levelling it - finally I buff it with for example the finest (11 000 grid) micromesh products, or even polish with polishing paste. This takes a lot of time. Before you start adding finish you can try to carefully sand the sharp edges of damaged surfaces to make them look softer because they can be very easily seen under the ready finish. If you have covered the top originally with nitrocellulose lacquer, you also can try to make the ding look softer by using shellac varnish - after that you add many layers of new lacquer. The last few layers should be put with spray gun. The dry season is the best time to glue the cracs.
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Re: Soundboard crack- glue(s) and repair humidity, Ding

Postby StevSmar » Sat Feb 18, 2012 3:22 am

Hello Marcus, Alan and Vesavekkuli,

Thanks for the suggestions on how to fix the cracks & ding. I'm going to cut some cedar this weekend and give it a try. I would have thought that I should wait until the humidity was higher but now it makes sense to fix the cracks while it's dry inside the house.

I have put an Oasis case humidifier in the case, it seems to bring the humidity up to 40-45%, when the house humidity is close to 30%.

I'll let you know how I make out, it will be wonderful to hear my guitar again.
Regards,

Steven from Winnipeg
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Re: Soundboard crack- glue(s) and repair humidity, Ding

Postby Tongdee » Mon Feb 20, 2012 3:58 am

The lid drop ding has broken fibres in the wood. It can be repaired by carving out a slightly larger patch from a similar grained piece of similar wood (even veneer)and splicing it in to the top as close as you can get the grain to match. I would sugest a diamond shaped patch, hide glue would be best. Scrape,sand it flush with surounding wood feather the new finish into the old.

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Re: Soundboard crack- glue(s) and repair humidity, Ding

Postby Vesavekkuli » Mon Feb 20, 2012 1:21 pm

About Tongdee's good suggestion: the lines on cedar can be then imitated with a suitable pen before adding finish.
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Re: Soundboard crack- glue(s) and repair humidity, Ding

Postby StevSmar » Sun Feb 26, 2012 8:45 pm

I have now installed all the cross-grain "cleats" on the underside of the soundboard. In a day or so when the last patch is well and truly dried I will put strings on the guitar and see if it holds together....... That's going to be nerve-racking.....

I haven't decided yet if I am going to attempt to re-finish the top, especially since I have suspicions that my guitar is going to crack further. While I'm thinking about that I'm going to "temporarily...." put pieces of low-tack clear plastic film, the type used to cover books, over the cracks & ding to stop dirt & moisture getting into them.

Tongdee & Vasavekkuli, thanks for the suggestion about repairing the ding. You were right that because the wood fiber's were broken that I wouldn't be able to get the ding out with water and heat. I think I will try one more time to remove the ding before I admit defeat.
Tongdee wrote:The lid drop ding has broken fibres in the wood. It can be repaired by carving out a slightly larger patch from a similar grained piece of similar wood


The crack above the fretboard (the left side) didn't close up and it's approximately 1.0 to 0.5mm wide. Once I find a bit of cedar that's close to the Western Red Cedar that the tops made of I will glue a sliver into the crack, and also patch the ding.

Working out how to mark the locations for the cleats was challenging. I cut a pencil and attached a piece of wire to the end and then bent it into a "U" shape. When it was inserted into the sound hole, my hand was then above where the crack was.

A co-worker lent me a video boroscope to look at the cracks with- it's didn't work as well as just using my inspection mirror and shining a flashlight onto it.

The suggestion posted by Dennis Tolz for using magnets to hold the patches worked FANTASTIC! I never would have thought of that. I also used the magnets to hold a steel-rule so I could take approximate measurements for the cleats.
http://www.classicalguitardelcamp.com/viewtopic.php?f=11&t=64738#p708660

I used scraps of balsa wood which were cut by trial and error down to the correct size for the cleats. I didn't find a successful way of putting glue on the cleats and not getting the glue smeared in the wrong spot on the soundboard as I adjusted the cleat into the correct location.

Thank-you for the helpful suggestions to repair my guitar.
Regards,

Steven from Winnipeg
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