1) Cut some wooden stringers to size, and wrap them in waxed paper, to which glue won't stick.
2) Fill the crack with a hide glue that will dry to a colour to match the wood.
3) Use deep-throat C-clamps to clamp the wrapped stringers to cover the crack inside and outside of the sounding board, to align the edges of the crack. The stringers should parallel the crack.
4) Apply pressure ACROSS the crack to force the edges together. (Wrapping the sound box with a bicycle inner tube does this excellently.)
5) When the glue has cured completely, remove the stringers and use a cabinet scraper to remove any glue squeeze-out from the exterior of the sounding board.
6) Sand and re-finish the entire sounding board.
I've used this method on a neighbour's mandolin, with good results; the seam was nearly invisible. The instrument sounded good, with thw added bonus that the glued seam helped stiffen the wood against string pressure.
Bill Rogers (Moderator)
Go to frets.com. Then the “Big Index Page.” In the “For luthiers” section, you’ll find ample information on crack repair.
I will add that there is a high probability that there are braces that are loose and possibly damaged.
Edited by - rcc56 on 06/22/2019 22:35:42
That looks like a solid impact on the bridge, loosening the bracing inside and driving the bridge downwards to generate those cracks.
I make ukes, and I'd fix it like this:
1. Reattach the bracing, making sure that the cracks line up when I glue the braces on. It's sad when you realise you've glued a crack in a wide open position! This will be tricky because you're working through the sound hole, but you might be lucky. There's a good chance (a) that this uke was assembled using hot hide glue, and (b) that the bracing is still partly attached. You should be able to tell the type of glue using a light and a mirror and scraping a bit off - HHG looks like semitransparent material and is brittle, synthetic glues are usually opaque and less brittle. If it's HHG:
a. Work out a clamping system that lines everything up - the bicycle inner tube could be useful here. You might need to make clamps to reach the brace ends, because it's hard to fit two commercial woodworking clamps in the sound hole. But with HHG you could glue one end of the brace at a time.
b. Work HHG into the section you plan to clamp, and clamp it all up. Heat the soundboard first to no more than hand hot (a hair dryer or heat gun works here, but be careful to keep a heat gun moving as it can scorch rapidly).
c. Once it is clamped, heat again. This reactivates the glue. Then snug up the clamps.
d. Remove any squeeze out you can reach once the glue has gelled, maybe 20 mins or so, using a scrap of wood. Once dry, a warm damp cloth will slowly remove the remaining squeeze out.
2. Once the bracing is fixed, glue up the cracks. Hot hide glue again.
a. Warm the wood, paint HHG along the crack, flex it to force the glue into the joint, add more glue and repeat. Wipe off surplus glue.
b. Now clamp the crack into alignment - rare earth magnets are really good for this, but you must protect the top with cauls because they snap into place. Re-heat at this stage to allow the magnets to pull the crack into alignment.
c. Then your bicycle tube to pull the crack closed. More heat.
d. Next day, clean up with a warm damp cloth.
3. Finally I'd cleat the long crack. I'd make a tool (a spike on a stick!) to place the cleat correctly, a dab of HHG and just hold it in place for a couple of minutes until the glue has grabbed.
HHG shrinks as it dries, so it's really good for pulling cracks together really tight. If you get everything lined up right the repair will be practically invisible and you won't need any finish touchup.
If the glue isn't HHG then you'll have to try to clean the old glue off the braces and soundboard, which is lots of sweary improvising using sandpaper strips! Then proceed as above using Titebond Original (not any other Titebond version) but without any heating.
The way I'd approach this is to remove the back, remove the bridge, and remove the broken or loose braces and repair the cracks. Then I'd reinforce the cracks with cleats and make an oversized bridge plate. These are ladder-braced and there isn't much to work with. I have repaired two of these with the same problem.
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