I posted a question about this instrument a few years ago and it was discussed briefly.  I have decided it is time to open it up and fix it and I was wondering about two possible options.

I have a 1936 Black face K-1 and is is excellent shape except that about 10 years ago it developed the dreaded top sag under the bridge disease.  -- a common ailment in these under built instruments. 

I did not play it long after it happened so the problem a this point the sag is "reversible".  That is when string tension is slack off the top rebounds to its proper carved contour.  I have looked at Frank's repair of the 1920 K-2 mandocello on this site and propose to open the back and make a similar repair on this instrument. 

I will not have to do all the heroic efforts that Frank did to return the top board of the instrument to proper shape however nor will I have to do any work on the end blocks.  So this is a much easier repair I hope.

So my question is this what do folks think of using an X-bracing for the top rather than the second transverse brace under the bridge that Frank used?  The X-brace idea is stolen from Bruce Weber's discussion of a oval hole Weber mandocello that he built a few years ago.


My goal is to stabilize the top and yet make a minimal impact on the K-1 tone/sound.  Would anyone want to wax eloquently on the pros and cons of these two approaches.

Bernie Daniel

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Hi thanks for that clarification!  I am leaning for reasons mentioned to just doing what Frank did. 

That K-2 he fixed still sounds awesome to me.  Did you happen to click on the video I included above?

The L-50 was a 1942 and yes it had a truss.  But it was broken off at the headstock just above the nut and badly glued with some flat metal supports screwed down-- i.e., it was a mess.  But I got it cheap.

So I took all the metal off, melted the glue (thank god it was not epoxy) and then cleaned it up and re-glued with right alignment this time with fish glue.  I took the guitar fret board off and then replace the truss with a new Stew Mac hot rod and then had Mark Keiser (repair guy extraordinaire)  route two channels on either side that went across the break and CF bars were inserted for added support. 

Then Mark milled 1/4" off the top and back sides of the headstock and we added a 1/4" plate of ebony on the top and a 1/4" plate of mahogany on the back and then the head stock was reshaped.  I could have done this but I do not have the tools that Mark does.

The head stock was bound, inlaid to pre-1940's Gibson design, and Grover mandolin tuners installed.

The fret board is a new mandocello board from a LMI blank (1/4" ebony) patterned after my K-1 (but two frets longer) and it has Evo wire guitar frets.  It is about 0.1' narrower at the nut than the original guitar board so I had to slim the neck down and re-finished -- Mark did that too.  My only regret is I did not go with 0.2" less than a guitar nut

I made the tail piece from a standard Gibson TP and some 1/8" brass round stock and silver brazed it together.  The TP cover was a custom job.

The bridge base is from an arch top guitar and the saddle is carved from ebony with a 1/16" thick bone top.

I actually go on the Mando Cafe a lot and have discussed this instrument.  But I posted here because of Frank's repair of exactly the same issue.  Thanks again for all the help as a result I am pretty close to being ready to go I think -- waiting for my spruce to arrive.  BD

Thanks for elaborating on your archtop conversion, very tastefully done!

I agree about the wall hanger status if the repair was reversed but I've come across a few people that think of themselves as collectors of "original" instruments and playability isn't such a issue for them. IF it's playable, all the better, but it doesn't have to be. There's a decent chance that they wouldn't know how to play it anyway.

Just seem silly to me. 

I'm glad you weren't offended, Bernie. The conversions look great!

 My biggest reservation about this sort of thing is that, in my opinion, getting the back OFF isn't the hardest part. I think getting everything lined up correctly to get the back on again is usually a bigger problem. 

Given the pretty "low priced" (trashed) nature of a lot of my projects, I think I tend to remove backs for interior repairs more often than most of the professional repairman. Basically, I don’t have much to lose and I’m not getting paid for my time so why not?  (Time is a great luxury when you don't get paid for your work.) Doing that usually becomes a very good lesson in the importance of the back as a structural element. My experience has been that the sides start moving out of shape before I even get the back completely removed. I haven't tried this but I pretty sure that removing a back then immediately replacing it will still require "wrangling" to get everything aligned again.

 Gibson mandolins are known for having “contrary” sides when it comes to getting them aligned with the plates. It's astounding how much pressure it can take to get a short section of side to move the last 1/16th of an inch to bring it under the edge of the plate. It could be that this will not be such an issue on the larger sized body on your instrument. I hope so, anyway.  

Even if it is that case, I think it would be helpful put some thought into how you are going to reassembly things before you disassemble the mandolin. One thing I’ve used is a set of “L” brackets which Frank has documented so well on his site.  

Another approach would be to build a form for the body before you pull the back then make some spreaders to hold the shape. In this case you probably don't need a form that's full height. I think a form cut from 3/4 inch plywood with a way to "lock" it around the body. Since you’re not removing the top plate you only need to support the sides near the back. If you do this and are careful about cutting it out, you can also use the plug from the interior of the form to make spreaders.   

When I’m starting a new project at least half of my planning is usually concerned with getting it all back together properly. "If it can go wrong..." applies to me in grand and terrible ways sometimes so I try to think ahead, as much as possible. Sometimes it even helps!  

It's completely possible that your instrument will not shift at all, but given the nature of this type of thing, I doubt it will be THAT easy. It is, after all, an old Gibson and they do have a reputation to uphold.

Anyway, that's my two cents worth. Since your instrument doesn't appear to need a lot of work reshaping the top, manufacturing a new brace or even building a totally new brace system shouldn't be too big a job if you take your time and think thing through.

You’ve got to love a hobby/job where even the "routine" stuff changes every time you do it, don’t you?

Wow that is terrific advice Ned! 

I was indeed aware of the tendency of sides to move once decades of stress containment are removed. And I have read horror stories about trying to glue a back plate on after a repair. I was thinking a ways to prevent it along the lines you mention but it is invaluable to hear exactly how the problem might be approached and contained. 

Systematic project pre-planning was never one of my best skills but this note is making me take a better look and hopefully I will go into the project with contingencies covered. (<:  Certainly the making a specific focus on the reassembly part of the project is great advice!At the very least you have increased my chances of success

I also just do these "projects" as a hobby and yes I love the challenge of every repair job -- it can be frustrating but in the end things usually work out.  Almost every thing I do takes twice as long as I had planned! Minimum.  Thanks again for the great post!

Ned is absolutely correct about the difficulties when it's time for the back to get glued on. Each instrument will have it's own personality, so even in spite of careful planning or clever jiggery, you will still need to adapt on the fly.

I have not experienced the sides being so much the culprit with the old Gibson's but rather the backs being a problem. Once they are released from the ribs, the old wood seems to want to relax and do it's own thing, 100 years of tension released. The last one I had the back off of for therapy, I built a rather elaborate jig to keep things aligned. It was excellent for a holding fixture for the entire process but I ended up abandoning it when I clamped up the back. These pictures say more than my poor typing skills.

Blocking added

A nice holding fixture as you can see.

Here is the alignment spike I mentioned.

This is the X - Y holding rods to help keep the sides pushed into the mold. I was diligent about assembling them back into place after every process was done for the day.

Here is the alignment pin again. I have no idea how long the alignment pin strategy was used by Gibson. This oval hole is a late teens model.

Here is a picture of my set-up getting ready to glue this thing up. The Infrared lights are very versatile for hot hide glue work!

I went for a dry run to rehearse my clamp-up and discovered that the jig was too unforgiving. I couldn't push or twist or otherwise manipulate the ribs into the shape of the back. The back was also a bit smaller now even though it was stored together with the body, it had a mind of it's own. I opted to hammer it with humidity with a couple of large Dampits in a plastic bag, a big help

At this point I decided to loose the holding fixture and do a free clamp-up with the neck held in my pattern vise.

I re-used the alignment pin, clamping the neck block first and worked my way around on both sides to the tail block. I did not put glue everywhere first and then try clamping all at once. I applied the hot hide glue a bit at a time, starting with a good dose at the neck block then prying the back open a bit,  add some more as I worked my way around and so on till done. I like the squeeze clamp, there very fast to align and clamp.

The down side to the clamps is that hide glue can stick to the pads enough to remove finish when you pull them. Be careful of this.

There where a few fingernail deep ledges after clamp-up, not much way around it when the back has slightly changed shape.

The hide glue is a breeze to clean up with a rag dampened with hot water. A bit of oil based leather dye color blending mixed with Linseed oil does a quite good job of coloring and disguising the slight miss-alignments.

I suppose one could bind the back after it was done. People would talk for decades about the rare K-1 with a bound back.

Thanks, Paul, I got so busy writing about forethought that I gave short shrift to actually gluing the back on again. The backs do sometimes get wonky, particularly the carved ones. 

 I've tried to make  pushing sticks so I can press sides out by going through the sound hole but the push sticks were just too much trouble to try to manage with everything else. I've only struggled with a couple of guitars that way and don't think it will work at all on a mandolin. 

On a particularly misaligned glue-up, I attaching the back in the areas where the sides were not spread wide enough and let it set before I tried any other area. Once it set up I was able to push on the edge of the back opposite that area to bring that side into alignment for gluing. I made a mistake at first because I didn't think about keeping the sides square and ended up with a back/side alignment what was even worse after unclamped that side.  I released the second glue up I was able to square thing and glue it up again, let it dry while holding it square and then continuing on what the rest of the back/side. I worked my way around the instrument gluing thing up a bit at a time that way until I got it done. It's a pretty slow way to do this and I constantly worried that I wasn't getting enough glue in the joint but it's held up well so far.  

BTW;  Not that you need my approval, but that mandolin looks really good to me, I always end up with some sections that just won't align perfectly and there's just not much you can do to correct a reveal on the neck block like that. My snakehead has a very small  reveal like that which I'm almost certain is a result of the back shrinking which is to say that it's probably not an indication of a repair. 

 At a personal level, I kind of  like the idea creating a future "rare" collector's item with period accurate binding. Of course even I wouldn't do that to a mandolin like this one.

I've had a Martin 12 string neck floating around for years. One of these days I'm going to "franky" up a 12 string by grafting that neck into a seagull body I got a while back. I figure I'll leave the seagull labels in the body and the Martin on the headstock then let posterity sort it out. I've always wanted a 12 string... just not enough to actually buy one.   

Appreciate the additional thoughts and description the process and problems.  After reading these post for the past two days I have a whole other view of what I might expect when I open this thing up!   LOL!

If I understood you correctly you are saying you glued the back down by sections-- how did you manage to work glue in between the sides and back board if you have already glued a section of the back down?

Bernie, it's a glue a bit process, clamp, wedge the adjacent area open slightly, inject the hot glue with a pipette while spreading it with a small thin palette knife and repeat. Even a stiff back will pry open enough to get some glue in.

With the piece heated up under the Red lights, you have some time. Also mix your glue a bit thin so it wicks more readily, this also extends open time. With hot hide glue, you can re-activate an uncooperative area with fresh glue, if things start tacking before you get it aligned. Four inches or so at a time or one clamps worth at a time. The project will dictate the length of each application by how well it yields to your pushing and pulling things into alignment.

Well Paul that is an amazing set of pictures!  The final result of the repair looks terrific.  I guess this was the one that had the top sag.

I have been thinking all day of what kind of a jig I was going to devise to keep the ribs from moving after the back was off.  What you used was one approach that occurred to me.  Seeing your version of it is very helpful! 

I was also considering making as set up along the lines of the aluminum L-brackets with the adjustable screws like Frank did.  Only I was going to use pieces of hard wood cut as a set of right triangles with an adjustable screw in stead of the aluminum L-brackets  I could put a caul shaped to match the contours of the ribs on the end of the adjustment screw. 

I will think more about the approach you took here also.  I'm surprise with all the prevention that you still needed to adjust the ribs when it came time to glue.  I can't see how they could move with that containment on both the outside and inside?. 

I am really surprised that the back board changes as well. Never would have thought of that happening!

After seeing all this I am really going to work to see that those sides cannot move after the back is off.

BTW this K-1 does have back binding already! (<:

These images are from a different mandolin but with the same problem and therapy as the one I originally posted.

"I'm surprise with all the prevention that you still needed to adjust the ribs when it came time to glue.  I can't see how they could move with that containment on both the outside and inside?"

The ribs shape and geometry did not change but the back did. In order to wrangle the back fit, as Ned put it so eloquently in a previous post, I had to loose the jig to be able to flex the sides. The back was no longer the same shape as the mandolin in the form holding jig.

The jig was wonderful though as a holding fixture to do all the the work to the mando with the back off. I don't think that the L bracket fixture would be quite as good of a holding fixture while doing back removal, lining repair from back removal, brace repairs and whatever else needed done while the back is off. 

I'm glad to see that you're not taking all of this as an attempt to talk you out of trying the repair. I think this is a very advanced repair and it's the sort of thing I might suggest that people with less experience should wait to do but you seem to be up to it.

I think that we, as hobbyist, have a great advantage when we can wait as you have done to do a repair. I like starting a new project with a little bit of concern about my ability to handle it.  If I waited until I was absolutely sure I could do it, I'd never start anything. Stretching my ability is part of the fun. It only works for me because I've developed a pretty good feel for the "edges" of my abilities. I had to learn where those are and then how to judge how far past that I can go and still get a good result. It's what keeps this fun for me. 


Now to the important stuff.

When I glued the back on in sections, I used a blow dryer ( for hair, not a heat gun) to warm the section I was going to glue then used a combination of a small brush and a thin pallet knife to "slather" as much glue as I could into the gap. I can usually get enough in most of the section with the brush but I like to make sure there is glue in the gap next to the previously glued area. That's where a very thin pallet knife come into play. I'm careful that I don't scrape too much glue off of the joint in the process of trying to get glue into the tight points but I want to have at least a little in there too. I get hot hide glue everywhere when I do it but I'm more interested in avoiding any glueless points. It's a mess but almost any glue can be cleaned up with the right approach. I haven't used this method on anything with "f" holes and don't know if I would because of the glue mess on the inside. I'm not so sure of reaching it well enough with the limits of access. I'll cross that bridge when I come to it.

To reach inside, I use various things but I've found that a steel cloth hanger cut with a barb on the end to snag on a rag  can be fairly handy for reaching deeper into the body.  I like using disposable shop cloths for glue clean up. If you use warm water and don't wring it out too well ( I don't like it dripping but I also need the water to make and keep the excess glue soft enough to wipe up) it's not hard to clean up the HHG. It's NOT quick but it will clean up with patience.


Hey Ned, here is a simple way to control glue mess inside - if you have the back off.

This is the same mandolin project. Paper or plastic with tape to mask the entire interior.

Would work with F-hole instruments too.


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