Closing of cracks

[Bob Steinarsson]

Dear luthiers

I have a question about repairing cracks in flat tops. I have read what I have found about this topic in this forum, but I have not found answers to my two questions.

Picture no. 1 shows a crack in a guitar that has been humidified in high rel. humidity (70%). After some days, the crack was absolutely gone. I then left it in a room with about 40% rel. humidity. After several days the crack looks like what you see on the picture. The string is 0.011”,  and the crack is at its widest approx. the same.

Picture no. 2 is of a Larrivee that had a similar crack, and that I treated in the same way. After some days in normal conditions, the crack opened a little, and I glued it with white glue and glued two small cleats over the crack inside the guitar. As you can see, the seam became black. It is very narrow, but it still is disturbing.

Then to the questions:

• In order to prevent the discoloring of the crack, I am tempted to humidify the guitar until the crack closes completely again and then glue two cleats over the crack without further gluing of the seam. My idea is that this would be enough to stabilize the crack forever. Is the crack too wide for this? At the widest, the crack is approx. as wide as the string. I am afraid of new cracks.
• Has anybody an idea of how to hide the black line in the crack on picture no. 2? The glue is “under” the top, so there will be space for some very fine filler.

Thanks

[Roger Häggström]

Use oxalic acid to clean the crack and then glue with hot hide glue. Have a look at this

https://www.youtube.com/watch?v=tViTWUL8Was

One problem with humidifying to close the crack, followed by gluing and cleating, is that you will have built-in tension in the top when the top dries. A new crack may appear beside the glued crack. If the crack is too wide you will need to fill the gap with wood.

[Bob Steinarsson]

That is actually my first question. Do you regard a 0.012"  (0,30) mm wide crack too wide for this treatment? See picture no. 1

[Bob Steinarsson]

The idea with oxalic acid seems very good. I'm happy to see that I everybody else living in Scandinavis can find it here: https://www.biltema.no/bilpleie/bilvask/felgrengjoring/felgrens-oxi...

[Roger Häggström]

0.3 mm will probably work.

[Bob Steinarsson]

Thanks.a lot

[Roger Häggström]

Looks like oxalic acis is pretty toxic! Don't drink or inhhale it and make sure to wear rubber gloves.