Mandolin neck repair: the final solution?

[Bernie Daniel]

Yes, on the caul. That is large area and you need really high force to clamp it together and move the extra glue out of the space. I would suggest start clamping in the center and quickly add clamps towards ends so the glue has somewhere to escape.


It's not about the fit of the piece alone. Any trapped glue can work against you. I've seen CT scan of old Cremonese violin with chest patch where thickish layer of glue was trapped inside. You can bet that the restorer fitted the patch meticulously to the (millions worth) violin, but glue can be your worst enemy on such large surfaces.

I understand.

You are getting at something John Hamlett has talked about and that is some glues have good adhesion but poor cohesion so the glue as a structural component is not reliable.

But maybe that won't be a problem on this job?

I am going to be using a "professional grade" two component marine epoxy to glue the carbon fiber bars into the truss rod slot on the other side of the neck. This material has very high adhesion, it has solids in it so it has very high strength, and dries hard and the dried material is highly resistant to flex. What is more it bonds to both wood and carbon fiber. It is heavily used in boat construction to glue wood boat hulls below the waterline.

So those two bars of CF that I will fit and glue into the truss rod slot will be "permanent".

Since I have the epoxy I am thinking about doing the entire job with it. Including gluing the fret board back on and also to gluing down this back strap?

Since this epoxy does have very good cohesion it will be less sensitive to the concerns of glue pockets glue because it can also be used as a structural material and a filler. For dang sure that joint will never fail as a result of heat or moisture.

In fact, as the tile of this thread says this is the "final solution". This is one job that I have no concerns over never being able to take it apart again. If this neck ever fails again it is trash.

[HoGo]

I would personally use HHG (without adding any rods or such), but with this area Titebond should be good choice as well. Marine epoxy will hold as well as no one will ever want to open that joint.
The problem I mentioned is about large surfaces being glued. You need to apply just enough glue, not too much. If you apply the pressure evenly on the whole surface at the same time with shaped caul, any extra glue near center of the surface will have hard time getting out so clamping the joint from center towards edges is essential to avoid glue (or worse air) pockets. With epoxy any air can be your enemy but that is much less likely to happen with slow cure epoxy.

[sunburst]

Bernie, what do you plan to do here?

I fear that the butt joint at the end of the piece, if left as is, will eventually be obvious no matter how it is dealt with.

[Bernie Daniel]

John it will be covered with a layer of ebony veneer (0.02" thick). I have a life time supply of the stuff. 😀

[sunburst]

If it's not too late to work some kind of scarf joint-like thing into that, the line of the joint will probably telegraph through the veneer rather prominently. (Knowledge gained through experience.)
PM sent, by the way.

[Br1ck]

Not to cast doubt here, but "The Godfather" was released in 1972 and Jerry Rosa didn't start building until a decade after that:

"Rosa String Works was born out of my desire to play and build my own acoustic instruments. Back in 1982 I couldn’t afford nor justify spending thousands of dollars on a mandolin since I was just a beginner. So I decided to build my own. I soon built another, then repairs started rolling in. The rest as they say is history."

https://rosastringworks.com/

Perhaps a sequel.

[Bernie Daniel]

Waiting for the bonding to happen. Eight clamps hopefully doing their thing.

[Bernie Daniel]

Clamping and bonding went well. Two pics: 1) fresh out of the clamps and 2) rough carving, the first session -- the easy part!

[Buck]

I really appreciate the progress photos. My guess is this will solve your problem for good. :-)

[Bernie Daniel]

I really appreciate the progress photos. My guess is this will solve your problem for good. :-)

Thanks! I hope you are right!

One more step getting closer now but still more fine carving and sanding needed. Enough of this fun and merriment for today though........

PS if you look close, I think I matched up the curl pretty close. At least I hope so.

[pops1]

Looking good Bernie.

[Mando&Me]

Nice work, Bernie. Lookin' good.

MM

[Bernie Daniel]

The back strap is done. So now it is time to work on the other side of the neck.

I inserted two carbon fiber bars that essentially fill the truss rod cavity. They are stack on top of each other and glued in with Jamestown Thixo marine epoxy so most of the neck has a double CF bar in it. The truss rod is gone of course.

The lower CF bar extends 1.5 inchs into the truss rod pocket and is flanked by two other 0.75 inch bars of CF -- then the entire cavity was filled with epoxy and then capped with a aluminum truss rod cover that is also glued down with the epoxy.

This epoxy dries rock hard and has a very high modulus of elasticity so it, along with the aluminum truss rod cover and the CF bars become part of the neck repair.

This should be a pretty strong neck?

Needless to say this repair is "final" and it will never be opened up again.

All you guys who like to be able to take things back apart will be grinding your teeth perhpas? But this neck has been repaired four times in the past. It was time for something different.

And don't worry I wiped the truss rod and head stock surface the isopropyl alcohol to remove any epoxy smears seen in the image.

After this cures the fret board is glued back on and for better or worse the repair is finished.

[Jeff Mando]

........this neck has been repaired four times in the past. It was time for something different.

One way to look at it --- besides playing music on it between the repairs -- the mandolin has provided a great repair hobby for you!

[Bernie Daniel]

Now the fret board goes back on. Waiting 24 hours for the epoxy to set.

Then make a new nut and string it up....

[Bernie Daniel]

Well the mandolin is back -- I think (hope).

Here it is freed from it's bonds and clamps hanging alongside my other Gibson F-5 imposter.

Still need to make a new nut, clean up some squeeze out glue (although most was taken care of by masking), and then fine sand and re-stain, install the hardware etc.

Other points of interest. The fret board is as flat as a pool table -- really nice. And it is light as a feather -- no truss rod.

Possible downside. Hopefully, the fboard is not too flat? I am counting on the string tension to produce a little relief - but not too much! There is no capacity for adjustment anymore -- again no truss rod. So I am not out of the woods yet!

I am kind of pleased that this rather unconventional approach to the repair actually came to fruition and turned out pretty much exactly as I had envisioned it. That is a rare event really.

Also thanks for the many helpful comments and suggestions from this forum -- I probably would have screwed this up without them for sure. I'll take some more close ups of the repair when I have time today.

The third mandolin to the extreme left in the pic is one I built earlier -- in case someone was wondering.

[Bernie Daniel]

Ebony veneer to hide the back strap on head stock. What a pain. Lots of fine detail work left then the staining will hid a lot. This is a 40 - 45 year old mandolin so I want it the repaired area to look distressed. Note on bank of tuners it right on the edge but I didn't do that!

[Bernie Daniel]

Back of the head stock has been sanded in Tru-Oil slurry to seal it.

The neck was re-stained with Trans-Tints. I'm not satisfied with it yet and the epoxy seems to have created a small area that I can't really stain well. But that is all I'm going to do now until I string it up a confirm that it will be playable.

[Bernie Daniel]

Just to "close the loop" on this thread so to speak.

For now the repair though slightly unconventional seems to have been a success. The mandolin was strung up with D'Addario J74s (yes I still have sets of those). The mandolin came up to pitch just fine. I set the action at 0.06" on both the G- and E-courses and measured the relief this morning at fret 7 at 0.006". It plays easily and sounds just as good if not better then before the repair.

The repair consisted of:

1) A 0.133" back strap of hard maple glued on with Jamestown Distributors Total Boat Thixo Marine epoxy. This is a "professional grade" 2:1 epoxy meant for wet and underwater (boat hulls) bonding. It is extremely strong and of course should not be melted by the heat or humidity issues a mandolin is likely to encounter. It is messy to work with like all epoxy cements but is very slow setting so that the worker has up to 37 minutes (room temperature = 70 degrees F) working time. Total cure to a very hard and inflexible solid is 24 hours.

2) Filled the truss rod channel with two bars of carbon fiber. The lower bar protrudes about 1.5" into the truss rod pocket and across the old break. The upper bar is the length of the channel only. These were glued in and also glued together with the marine epoxy.

3) The truss rod pocket was filled with epoxy and bits of carbon fiber bar and the capped with a custom-made aluminum truss rod cover. This THC is now "permanent" but it is shaped so as to butt firmly up against the nut/fret board and thus it becomes a structural part of the repair as it crosses over the old break line on the top (like the back strap does on the bottom).

4) Fret board glued on with marine epoxy.

To be sure I am not necessarily recommending this approach to anyone at this time. If all is well in a year I might just bump this thread and let you know how it is doing. If if fails I will also let you know.

One again thanks to all who helped out!

FYI the gray spot on the lower part of the head stock is bondo that I used to repair the cover plate broken with the neck - it will be colored black later.

[Br1ck]

Well done and a gutsy solution.

[Mando&Me]

Have been following this thread with avid interest, since I will be attempting something similar in January.

Thanks So Much!

[Jeff Mando]

Good work, Bernie! I think you've got it!

Just curious, you said your mandolin was not a Gibson and you have had it a long time. Do you know who made it? I noticed a label in your picture, but couldn't read it.

[Bernie Daniel]

Good work, Bernie! I think you've got it!

Just curious, you said your mandolin was not a Gibson and you have had it a long time. Do you know who made it? I noticed a label in your picture, but couldn't read it.

Thanks for the thoughts. I don't know who. I have two of fake G-word F-5s (made by different individuals) supposedly in this (OH, KY) area. I have some clues about who made the other one but not this one. The label you saw in the pic is also fake L-word

I was glad this repair worked (I hopefully permanently) because this is a really good sounding mandolin. Seriously, it's right up there with a modern Kentucky KM-1000 or such -- that level or more. Who ever made it knew what they were doing or really lucked out. Anyone could play this mandolin in a bluegrass band and not have any issues with tone or volume - at all. In fact, it is on the loud, woody side of things mandolin. I have to figure out what to do with it now - LOL. As I really have a two good Gibson F-5s -- more than filling my needs these days. Some kid starting out playing bluegrass and wanting a mandolin with mojo that is strong and plays well should have this mandolin!

So if the repair holds up for a year I will probably just sell it somehow with full disclosure of course. But seeing the strength of this glue (see below) and knowing that the bonds are essentially "impervious" to heat and humidity (heck water!) this mandolin should last longer than anyone reading this post?!! LOL

I did a lot of experiments with the epoxy before trying to fix the mandolin. It is a one part polyamine to two parts epoxide resin so I was very curious as to how sensitive it was to exact mix ratios. Also the glue ratios are 2:1 by volume -- don't know what that translates into by weight. I kind of assume the density of the two components is the same but did not actually know that. The manufacturer could not supply me with that information either. In the end I assumed equal densities and did initial trials with 2:1 by weight mixing.

IMO, even if the bond is not hyper-sensitive to the mix ratio you want to be as close to actual chemical stoichiometric ratios as possible so that you do not have quantities of unreacted epoxide or polyamine floating around in the bond -- for lots of reasons. Anyway, bottom line this glue is just like any other epoxy glue and it is easy enough to just eyeball the amount of the two components then mix and do the glue up. Once if fully cured (24 hours) the bond is incredible.

I glued two pieces of scarp maple together in one test and there seems to be nothing i can do to break that bond. If I put one end of it in the metal vice and used a long enough lever (piece of pipe) on it i suppose I will either break the wood or I'd manage to tip over the bench. LOL. No interest in doing that. This mandolin should be permanently fixed.

There are down sides. For example if you ever wanted to remove the fret board you'd probably have to mill or sand it off etc. But probably no need -- the fret board is ebony and on the thick side so a leveling and new frets is all it is likely to ever need? Right now its "perfect".

[hank]

Great save Bernie! I think she looks authentic to the untrained eye. Those screws on the truss rod-less cover add to the look but will mislead future I want to look in here types. How does the response and sustain feel now that she’s back in service. Can you tell any changes in behavior of your patient since the operation ?

[Bernie Daniel]

Great save Bernie! I think she looks authentic to the untrained eye. Those screws on the truss rod-less cover add to the look but will mislead future I want to look in here types. How does the response and sustain feel now that she’s back in service. Can you tell any changes in behavior of your patient since the operation ?

Thanks! I think it sounds a little different but I really don't trust my tin ear for things like that.

It has always been a very good sounding and playing mandolin and I think it still is. It feels a little lighter to pick up now (metal truss rod replaced with CF?).