Best way to fix this bowed neck?

I've recently found an old bowl back mandolin that is in serious need of repairs. Most importantly, the neck has a pretty aggressive bow in it. The action on the 12th fret is 6mm, the nut is barely 1.5mm high and the bridge measures 7mm in height across the middle. Without a bridge altogether (running a ruler from the spot where the bridge sits to the nut) the action would still be about 3mm. To add to this, it seems someone attempted to repair the instrument at some point. My guess is the neck suffered a crack where it joins the body, coincidentally, where the belly of the bow is most pronounced. I imagine I have a few options here:

A. Carefully undo the sloppy neck job (maybe using a heat source to free the glue), clean up the mess, and apply a new neck fix that also corrects the action. I am unsure of the type of glue used, maybe someone here can tell what it is

B. Remove the fingerboard completely, slowly remove material from the glue side to correct for the action, cut deeper slots in the nut thus reducing the overall thickness of the neck

C. Build a jig to hold it in place as I apply heat to the entire neck and proceed to clamp it down

I am not a professional luthier (as you can probably tell), and own a small collection of hand tools (nothing sophisticated). I'm not opposed to purchasing tools for the job, within reason. Here's a picture of the mandolin

Attachment 192468

Thank you in advance! (apologies for the bad photography skills)

Generally very few, if any, luthiers will jump up and say they do neck repairs on bowlback mandolins. You pretty can do whatever floats your boat. These weren't built like any modern day instruments.

There’s lots of info on bowl-backs on this site, but you may do better with a Google search and find some videos. Just a few days ago the topic of neck attachment came up here. You need a good mental picture of how these things are constructed before you start in. If the neck actually has a crack or is just separating from the rest, as Mike just mentioned, it’s a really big deal, because it isn’t neck + body, but more like body built on neck.
Whatever, you have to stabilize the neck or whatever you do topside is just a waste of time as it will continue to fold up.
Other than that, please post a photo of the front and back of your prize. Most bowl-backs aren’t worth much, but just in case you have something that’s rare or very interesting, someone here will let you know, which may mean the difference between an agricultural repair and something more expensive. (Posting photos here takes a little practice, but it works.)

I have had some sucess using the techniques shown by Dan Erllewine in some VHS tapes I have. Get the neck to relax and then re-fret with frets that have a fatter tang. This may not be aprops to your situation if there is a break or other problem. I agree with Richard500, more images and more research. Good luck. I am interested in your progress.

Maurice

My technique for repairs of this type are to remove the fingerboard, clean up the glue joints, re-glue the fingerboard with hot hide glue, using a caul that induces just a tiny bit [perhaps 1/32"] of back-bow, and finally a good tight compression fret job. The back bow will pull out after the instrument has been under tension for a while.

I didn't expect so many replies! Here are some more pictures of the instrument, in case anyone can guess when/where it was built (there are no obvious markings or stickers anywhere).

The butterfly on top was glued on with the same glue used in the neck job
Attachment 192473

Attachment 192471

Attachment 192472

The peg heads seem to be built of wood
Attachment 192470

My best guess as to its origin is an early 1900s kit someone put together. I will do more research on the construction of these to familiarize myself with them.

Hi mregger, it looks like a German mandolin of the 1910-1920, but does the headstock belong to the same piece of wood of the neck or is it glued to it? Because Neapolitan and Sicilian makers, unlike Germans, used to make a separate headstock and then veneer and attach it to the neck.
As for the bowed neck, you could check this website http://www.mandolinluthier.com/Hneck.htm
Cheers, Nico

You need a shorter straightedge that stops before the kink at the heel to see if there's really a bow in the neck or if the real problem is at the joint. You will likely need to remove the fingerboard for access to the neck joint, and inducing a slight back bow when you reglue it, as rcc56 suggests, will be adequate. It's not a good idea to remove material from an already weak neck.

While it is hard to tell from that picture, it looks like the neck has pulled up a the joint. I am guessing someone tried to repair it by simply flooding it with glue. As Greg says a shorter straight edge will confirm which it is.

It is uncommon that the neck itself has bowed very much, if at all. What almost inevitably happens is that the glue holding the ribs to the block inside the body, which in a European mandolin is usually an extension of the neck shaft itself, gives way and the neck pulls up. This often also leads to deformation of the soundboard. It looks as so someone has tried to re-glue the neck body join, as well. The fretboard need to come off and the neck pulled back into line with the plane of the soundboard. This may involve disassembling and re-gluing of the neck block area. It can get quite involved. Good luck!

My impression from the pictures in your second post are that the fingerboard is not original to the mandolin. The cut at the end of the fingerboard is shaped for a round sound hole, and is too short for the mandolin.

There is no way for me to say for sure without the instrument in hand, but I suspect that the geometry of the neck and the neck joint were thrown off kilter when the fingerboard was installed. I recommend putting a straight edge along the joint between the neck and fingerboard. If that joint is not reasonably straight, the instrument cannot be set up to play well without correcting the geometry.

Updates:
I've managed to remove the fingerboard by applying heat to it and slowly wedging a spatula in. This caused some minor damage to the soundboard unfortunately, but nothing too serious. The neck portion was clearly glued on at some point as you can see by the pictures

Attachment 192592

Attachment 192593

I'd have to remove the soundboard to be able to see the if the neck block and neck are built with the same type of wood. As @rcc56 mentioned, it's likely that the fingerboard and neck came from a different instrument. My next move will probably be to straighten the neck and put the fretboard back on. In response to @nico verde, the headstock seems to be the same kind of wood as the neck, however it was re-glued by the same person who did the neck-body job. Here's a picture

Attachment 192594

Looking at the neck wood without finishing, it seems like it's made of pine. I never heard of a neck built of pine... is this common practice? Thank you again for all the responses.

Many Neapolitan mandolins have a core of lighter wood overlaid with a hardwood darker veneer. It often looks like the neck was broken at that point but as nico verde says above, "Neapolitan and Sicilian makers, unlike Germans, used to make a separate headstock and then veneer and attach it to the neck."

In any case, someone truly messes with this. However, from what I can tell this is a lower-end model and certainly would be fine to experiment with. I would not sink any great amount of money to fix it but if you wanted to do it yourself it could be a decent playing instrument. I agree with the assessment of the experts above.

If you straighten the neck by planing it, just make sure that the eventual tilt with the fingerboard and strings allows sufficient height over the bridge, as there’s hardly any leeway on a bowl-back, and maybe a little more if the neck is going to collapse further.