Fish glue
by claesgellerbrink
My newly glued Levin LT-16
My dad’s old Levin LT-16 has quite a few crack, well it had, because now most of them are fixed. I tried to figure out what type of glue would be best for repairing wood cracks and all the articles I read said the same thing, warm hide glue. The only problem is that I didn’t feel too intrigued by the idea of having to have melted horse hoves and other animals parts on the stove so I was desperately seeking for an alternative. Then my new found guitar building friend Roger in Sweden told me about fish glue. It has the same awesome properties as warm hide glue but can be used cold. It took some time to locate someone selling fish glue but eventually I found die-moebelwerkstatt, a German eBay seller specialised products for furniture restoration. I got myself some Canadian fish glue made from a recipe from 1870 and it’s amazing. I practised on my Francisca Montserrat and some other old guitars that had cracks in them before I dared to touch my Levin. It has gone pretty well so far, no major difficulties, but I wished that the glue pulled a bit more. I read that the special thing with warm hide glue, and apparently cold fish glue too, is that it is pulling the wood together when it dries, hence why it is so good for cracks and similar repairs. It might be my glue that is a bit old, or just the recipe from 1870 not being the best, but I wished it pulled a bit more. It still works really well, way better than I expected. I feel a lot safer now, not only when it comes to buying old guitars that might have a crack or two, but to dare to use mine because if I happen to crack them I can always repair them with my fancy new fish glue.
Claescaster 
I see that you have tried the fishglue 🙂 If you want to test if it really contacts when it dries, try putting some glue on a thin piece of quartersawn wood. If everything is like it should, the pice of wood should bend into a half moon shape.
To REALLY do a great repair, you should glue a square piece of wood over the crack on the inside. Use quartersawn spruce, 2-3 mm thick, and make a small “pyramid” with thin edges. Ca 3 cm in a square. Make sure the wood grain goes across the crack and not in the same direction. That way the repair will be stronger and the crack will stay closed.