When repairing old instruments, cracks must always be glued together. All the cracks in the lid and the bottom I glue with warm skin glue which is simply the best glue for wood. I do it in two sessions.
First, the crack is glued together edge-to-edge and then it is important that the two sides are level with each other and that the crack becomes as tight as possible. Wide cracks must get a wooden stick glued, but that's another story. A glued hairline crack never learns to open up, if it breaks it happens next to it. But skin glue is not good at filling in voids and a slightly wider crack does not become as strong as if the wooden surfaces are completely in contact with each other.
To reinforce even the cracks that are not hairy, a so-called "layer patch" is glued to the inside of the instrument. A layer patch can be of different types of wood, spruce is perfect for spruce lids and light bottom wood such as maple. The grain in the spruce is important, you must make sure that the grain goes perpendicularly or at least at 45 degrees to the crack. I use mahogany as a substitute for dark woods (mahogany, rosewood, etc.). The principle is that the large adhesive surface of the small piece of wood (in relation to the adhesive surface of the crack) should help and withstand the next time the lid dries.
A traditional patch is a small square or rectangle cut into facets into a small pyramid. When a crack is to be repaired through the sound hole, it is one that must be practically used. Since I have the luxury of having the bottom open, I can use strips that cover the entire crack instead. I always use such strips on the bottom wood (unless the crack is very bent). In lid, I cut the strips into smaller pieces so as not to tie up the lid too much and give instructions for the next crack at the edge of the strip (see below).
Making team lists is not easy. I made several attempts to use a sharp planer, knife, grater and sandpaper on a block. For the most part, it failed when the thin and brittle spruce strip cracked - in addition, it always ended with plasters on the fingertips! Thought of and made a Macgyver jig with a piece of cut ruler, a clamp with wing nuts, a sanding block and a vacuum table to suck up all the sanding dust with (cough!). The sanding block is practically attached with a wooden plug and a little extra self-adhesive sandpaper sits on the jig itself. The sanding block has a countersunk piece of coarse sandpaper in one edge (masking tape has shimmed up to the correct sanding height).
Needed team patches in darker wood than spruce and cut into a bunch of 2 mm thick and 1 cm wide blanks of a mahogany lattice.
The jig was clamped to the practical "shelf" in the jig and a vacuum cleaner hose was inserted into the side hole in the vacuum chamber. All small holes in the ruler then suck up the sanding dust very efficiently.
Five pieces were put in place and were tightened with the help of the wing nuts. A counter bar under the plywood frame allows 5 mm to protrude. A thin strip under the plywood strip at the top ensures that the material is pinched.
Then just start grinding! The coarse sandpaper is effective and there is no risk that the substance will crack in the weak vein. Only the two outer wing nuts need to be tightened so that the blanks cannot slip. You grind until the block starts to go easily.
Then you turn the blanks and grind the other side. Remaining becomes a perfect triangular, lightweight and low bearing strip 🙂
Some pictures of the jig apart when I was done with all the mahogany stuff. Here you can see the bar and the extra strip on the plywood strip.
This is what it's supposed to look like!
As for the team patches in the lid, I took some pictures to explain how I do. I start by cutting a piece of strip in spruce of 2 cm, very simple with a knife.
Then flips off the ends to a tip with a regular pliers.
I want to break up the team list in the lid so as not to make the lid stiff unnecessarily and make sure that not one edge ends on a slight vein in the lid as a guide for the next crack. The picture explains the principle, the dash simulates a crack. If you have a very bent crack it is easy to follow it too.
It's too rare I need to use the jig (really fun!). I make the strips in batches and then store them in a cardboard tube that is produced when needed. The spruce moldings are made of halved purchased cover strips for the bottom seam. Once glued in place, it is easy to polish the patches to make them softer in shape. With hot skin glue no force is needed, you let the glue do the job as it sucks the pieces of wood together when it dries. The fact that the tips are high does not matter, it is the sides that have to be thinned out to nothing so as not to be a guide for a crack that always happens along a vein.