Regluing a loose back

[Mark Marino]

I've always thought about tinkering with instrument repair, and probably ran across an ideal test case to see if I have the patience. #I acquired an old beater strad-o-lin with one problem- the back came unglued around where the tailpiece attaches.. but once it came loose, the side started to pull up/out a little bit, so now it doesn't exactly line up with the back. #I'm thinking of making a form with some clamps to 'push' the body back into form and then glue the back, but am wondering with a 60 year old instrument whether that might cause other cracks or failures. #Any suggestions on how I should try to correct this? Would it help to humidify the instrument ahead of trying to move the wood? #Thanks in advance!

[PaulD]

In my amateur repair work I haven't had to do this, but here is a Frets.Com article that may be of help courtesy Frank Ford.

pd

[sunburst]

Proper humidity is a good idea any time, but if your shop is already properly humidified there's no need to humidify the instrument. Only if the instrument is very dry does it need to be humidified.

Compressing the rim to conform to the back is the only way to get the glue joint to line back up again, and doesn't seem to cause any unmanageable stress on the rest of the instrument.
Frank Ford did a test here and found that the rim can "relax" back into it's original shape pretty fast.
You can also read about Frank's method of clamping the sides in, there at Frets.com.
I'll be back in a little while with some pictures of a couple of the jigs I've made to do the job.

[sunburst]

OK
Here's a jig I made to compress a mandolin rim back back into position for gluing. I originally made it for an A-style mandolin, but later I needed it for an F, so I cut away part of the jig to make room for the points and scroll. It's just a scrap of plywood and some scrap lumber put together with screws and lined with cork for padding when clamping.

[sunburst]

Those are just wood screws, with their points ground blunt, going through the sides of the jig to use as clamps to compress the rim.
I always keep some of these cut-offs from mandolin sides and use them to put between the blunted wood screws and the rim of the mandolin.

[sunburst]

This mandolin doesn't have a loose rim, it's just one that I have in the shop and I'm using it as a "model" for these pictures to show how the jig works.
If there's not a screw hole in the jig where I need it, I can drill another hole and put a screw through anywhere I need one to push against the side to get it aligned.
I've just got four screws holding this mandolin in the jig for demonstration purposes. If I was compressing the rim I'd have a lot of the screws tightened against maple scraps against the rim.

[sunburst]

Here's a cork-padded caul that I can use to clamp the rim joint back together after it is aligned in the jig.

[sunburst]

Here's how it works.
The wood screws compress the rim until it conforms to the shape of the back, and the clamps and caul clamp the joint while the glue dries.

[sunburst]

The front of the mandolin is clamped all around the edge against the cork-padded surface of the plywood which acts as a caul to evenly distribute the clamping pressure.
One of the advantages of this jig is portability. I can pick up the whole thing and lean it in the corner, put it on a shelf, or anywhere off of the bench and out of the way while the glue dries.

[sunburst]

One of the disadvantages is; it only fits mandolins. When I had to glue a loose mandola rim I had to make another jig!

[sunburst]

For other ideas on how to compress a rim to conform to a back, here is a look at a guitar repair.

[Mark Marino]

Paul & John, Thanks for the great tips. The jig is pretty much what I pictured, so I think I'm on the right track! I really appreciate your time in the detailed reply John.

[Mark Marino]

Everybody, thanks again for the great instructions. One more question... the back is only free from the side about half-way to the neck heel. Should I totally break this joint before regluing, or just try to get glue in the crack where I am able?

If I should remove the entire back, any tips on how to prevent other damage? I'm figuring a putty knife would probably do it.

I'll post the results when finished. I hope to have a playable Strad-o-lin as a spare when done!

[sunburst]

That's a "judgment call".
If the rest is securely glued, leave it. If it's not still secure, take it loose and re-glue it. Obviously, I can't tell "over the internet" whether it's still secure, so it will have to be your judgment.
Whatever caused the back to come loose where it did may or may not have affected the rest of the glue joint, depending on what the cause was.

Careful work with a thin putty knife will work, if you have to remove the back, but a palette knife might work better, being thinner, generally. (You can find them in art stores.)

[Andy Morton]

Hi--am reviving this old thread. It has been very useful. I am a hobby luthier and and helping out a friend who had the back of his mandolin at the tailpiece separate from the rim. The sides "bulged" out from the back---pulling some of the kerning loose. Since the kerfing was loose -- I decided to remove the back and reglue the kerfing (with HHG). I built a jig as shown by Sunburst (thanks John!) and its going well so far. I have sanded the surfaces (with a large sanding board on both the back and the rim surfaces) The surfaces are clean and there are no traces of the old glue. with some pressure applied using the "Sunburst jig" in the tailpiece area--about a .10 inch needs be pushed in to achieve good alignment in the tailpiece area.

A couple of questions/thoughts:


This is an Eastman 305...it had no locating pins in the headblock or tailblock. Would you put some in locating pins?

In terms of the glue up--would it be best to glue the back to the rims in one section at time. that is glue the part that mates without pressure, wait until that is secure and then glue the part that needs pressure . I understand that this would be similar to how violins are reglued using a pallet knife and HHG.

The back does not have any binding. I have thought of gluing it back together with minimal pressure to correct the bulging and leaving something of a mismatch at the tailpiece and then installing binding to deal with the mismatch of the rim to the edge of the back---thoughts on that?

Since you have to wrangle the rims inward with the jig.....are the chances good that this will delaminate again-- but I read (Frank Ford's test) this technique works pretty well.

Thanks for any advice--and thanks again Sunburst for sharing info on the jig you made.

[Andy Morton]

Here is a picture of the back and rim---and showing the mismatch. This is not glued up--the back is just resting on the rims here.

[rcc56]

I usually glue the neck and tail block first, lining them up into their original position. Then I glue the ribs, lining them up as best as I can. So far as I know, this has always been the standard practice in both the fretted instrument world and the violin world. If the tailblock is glued last, the neck angle will probably be reduced considerably from its original position. The decision to add binding waits until after the glueing has been completed.

[sunburst]

If the back is shrunken and will no longer align all the way around the rim, I normally "average it out" so that there is a small amount of misalignment all the way around. It is usually not very noticeable at all when done.
You shouldn't need to add binding, and I wouldn't advise it. That is a big job and gets bigger when you have to touch up the finish.

[rcc56]

I will add that each job of this type needs to be evaluated individually, and there are no hard and fast rules about the "right" way to do it. You are looking for whatever will produce the best end result. I have just edited my previous post to say "usually" rather than "always."

You might try dry-clamping the instrument a couple of different ways, and see what technique that suggests in this particular case.

I have found that it is harder to pull sides in than it is to push them out.

[Andy Morton]

Yes thanks. everyone for the advice. Very helpful!!
I built a jig similar to John's to add moderated pressure to bring the rims into alignment with the back and glued the area near the tail piece. Checking after the glue dried--the other areas seem to align pretty well with a small amount of misalignment but the main area near the tailpiece looks pretty good - Now that I know that the other areas besides the tailpiece are aligned pretty well--I will hot knife in hide glue (as violin luthiers describe) one section at time for the rest of the instrument (the entire instrument will be clamped, but the sections to be glued are unclamped for gluing and then clamped for the glue to harden).

Andy

[Andy Morton]

Here are some pictures of the jig (similar to the "Sunburst Jig" -- thanks John) I built to push the sides in to align better with the back:

ck:

[ATTACH=CONFIG]181394

[j. condino]

Here is my variation on the rib-o-matic. It is so common for teens and 20's A models that I have a dedicated jig that gets a lot of use. Also shown is a larger guitar sized model for a Collings that got left in a hot car last summer....

j.

[Br1ck]

You guys are making me feel so inadequate. Great thread.