Below are some photos of repair operations in our shop. They don't need to load for you to click on the links at left.
When a top crack passes over the sound post area it is the usual procedure to inlay a patch on the inside so that the pressure from the soundpost will not reopen the crack. In some circumstances we just glue the crack and hope it holds. This would be the procedure on an inexpensive instrument. The instrument below is a 1927 violin by Kurt Gutter, one of the better Markneukirchen makers, and it is well worth the time and expense of this sophisticated repair.
First, the crack is cleaned out and glued (not illustrated). In order to repair a soundpost crack, a perfect counterpart must be made. This is easier than you might think. First, a 3 cm. thick piece of pine or basswood is roughly cut to the shape of the violin between the c-bouts. It is hollowed out to very roughly match the arching, and then put aside. The violin is then covered with aluminum foil, as in the photo below.
A generous amount of Bondo autobody filler is mixed and deposited on the violin (the reason for the foil), completely covering the area between the c's. Then the piece of wood is placed over the Bondo so that it oozes out, covering the area to be clamped. The Bondo cures very quickly, and you have to watch the temperature, since this is an exothermic reaction. Usually the mold is hard enough to remove before it gets really hot. Within an hour, the Bondo has cooled and can be used. The foil is removed from the violin top and the sticky stuff that held it on is washed off. The Bondo counterpart is corrected, if necessary, and lined with thin plastic wrap. The violin top is clamped in place, with the camp handles acting as support legs, allowing access below the counterpart.
A template is used to lay out the area to be excavated. A gouge and scrapers are used to create a uniform trough to within .5 mm of the exterior of the top. That's why you need a solid counterpart. Now the new piece of spruce is prepared. It is split from a seasoned block that matches the grain characteristics of the top. The outside diameter is cut using the same template that was used to mark the trough, but it is cut about 1 mm larger. Rough fitting is done with knives and planes; then chalk is used to attain a more precise fit.
Final fitting is achieved by gluing studs around the perimeter of the trough and pressing the patch directly downward to pick up the chalk. Once fit is achieved, the chalk is cleaned out of the patch bed and from the patch. By shining a light behind it you can see how thin the patch area is.
The top 2/3 of the patch are split off, but the top piece is left in place to help spread the clamping pressure. The studs are left in place and the patch is glued with hot hide glue. After a couple of hours, the clamps are removed and the top is removed from the counterpart in order to clean up any glue that has oozed out. Then the whole thing is reassembled and the clamps replaced. It is allowed to dry overnight.
After about a day the patch is planed down to the original thickness of the violin. There is no evidence of the patch on the exterior of the instrument.
Wing Crack Repair
One very common problem with violin family instruments are wing cracks. They occur at the top of the bottom hole of the f's, usually due to too much pressure on the top while cleaning. Often they are neglected for years, and by the time the decision is made to fix them properly it becomes a monumental task to clean out the dirt and coax the edges together. Once they are glued it becomes paramount to make sure they don't open again. This simple repair takes the stress off the crack by replacing most of the wood in the crack area. First a counterpart of the outside is made with heat-softening dental compound backed with wood placed in a plastic bag so it won't stick to the varnish. The counterpart is clamped into place and a trough is made along the crack with a 5mm incannel gouge. A scrap 5mm soundpost is split and glued into the trough and, after the glue dries, it is trimmed flush with the surrounding wood. After this the crack is retouched, and, hopefully, it becomes invisible.