Caving Top Repair

[gemini1]

I have a 1936 Kalamazoo KM-21 which I am trying to restore.
The top is caving, in the area between the bridge and top f-hole, sloping
to a max indentation of about 9/32 in. at the f-hole.
There is also a straight 2" crack from the end of the f-hole to the binding
near the neck.

Please advice how I can restore the top contour and fix the bracing?

Thanks,
Tim

[sunburst]

You might be able to "jack" the collapsed area of the top back to it's proper height with a "hockey stick", working through the f-holes. If the wood is too distorted and "set" in it's current position to move enough with force, you might be thinking about whether the instrument is worth "sand bagging" the top. Frankly, few instruments are valuable enough for that.
If I've used terms you aren't familiar with, try googling them or just ask me what the heck I'm talking about!

[gemini1]

Thanks for the reply.

I can picture the hockey stick technique, but please clarify sand bagging.
Would a little heat or steam help, as the wood seems fairly set.

If I can restore the height, would I just glue the existing X braces,
with a temporary internal prop holding the braces while drying, or would additional bracing be needed?

[sunburst]

Sand bagging is a technique used by violin restorers, and borrowed occasionally by mandolin and other carved top instrument restorers.

The top is removed from the instrument and the low, sunken part is filled in with beeswax or modeling clay to the original shape. The top is then cast in plaster of Paris to make a mold. The wax or clay is cleaned off of the top and the top is placed in the mold that was cast from it. Heated sand bags are then repeatedly places on the inside surface of the top and clamped in place. Eventually, the heat and pressure conforms the top to the mold. It can then be re-braced and reunited with the rest of the mandolin.

The reason this is normally reserved for very valuable instruments who's originality is important are obvious from that brief description. I've done it to a couple of old Gibson mandolins, but only because the owners placed sentimental value on them above and beyond the intrinsic value and decided it was worth paying for. (Replacing the instrument would have cost less.) I modernized the technique a little by clamping a heating blanket between the sand bag and the top so repeated heatings of the sand were not needed.

I can't tell you anything about the braces because I can't see the mandolin. Sometimes re-gluing existing braces can be done, sometimes they need to be replaced. Sometimes the existing braces are enough, sometimes more need to be added.

[nkforster]

If the sand bag method is appropriate, then you may find it easier just to remove the back and have a deeper box in which to cast to allow for the neck and fretboard. Even better - remove the neck too. Then when the cast is dry, carve away the sunken part of the casting to the shape you would like it to be. Remove the bracing, and press out the top. Rebrace. Lots of work. Probably far more than the instrument is worth I'm afraid. Good luck.

http://www.nkforsterguitars.com