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:
Use oxalic acid to clean the crack and then glue with hot hide glue. Have a look at this
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.
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
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...
0.3 mm will probably work.
Looks like oxalic acis is pretty toxic! Don't drink or inhhale it and make sure to wear rubber gloves.