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Bridge reglue questions
http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10137&t=54631		 Page 1 of 1
Author:  joshnothing [ Thu Dec 16, 2021 8:01 pm ]
Post subject:  Bridge reglue questions
I mentioned in the ‘dumb stuff’ thread that I’d make a separate thread to solicit advice on a bridge reglue. Short summary is the customer left his guitar in the bed of his truck, under a black vinyl cover on a day where temp in the shade was 98. Bridge removed itself in protest:

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It was total glue failure, with clean separation everywhere except one wing and near the pin holes. Theres’s the usual issue with the factory using only part of the total bridge footprint as glue surface. The big slot routed in the top is to mount a transducer element directly to the bottom of the saddle. Seems to me it severely weakens the area and removes yet more valuable gluing surface. Not a great design IMO but there’s a ton of these out there in service and really this failure comes down to excessive heat exposure.

Pretty straightforward reglue, but I need to deal with this belly first:

Image

Can’t quite get a pic that shows it but the bridge plate is thinnish and made of plywood. Soundboard is around 1/8” thick. Can’t see any loose bracing on the inside.

So onto my question:

It would seem worthwhile to try the TJ Thompson belly reducer as a first measure. I’ve only ever used this tool in a context where the interior and exterior cauls exactly matched the bridge plate/bridge footprint, ie on the Martins for which it was designed.

None of the Thompson interior cauls closely match the bridge plate on this guitar.

My idea would be to use one of the cauls which is a little under size for the bridge plate and do a several treatments, shifting it around slightly to try and flatten the whole area.

Anyone tried this? Any pitfalls I should be wary of?

Would love to avoid having to try and pull this plate with its weak narrow section between pinholes and pickup rout. It looks like it would be easy to crack…
Author:  Chris Pile [ Thu Dec 16, 2021 8:29 pm ]
Post subject:  Re: Bridge reglue questions
A warped plywood top will remain warped in my experience. The glue between the plies has moved and reset while under tension and heat. I never could get the hump to reverse itself, and believe me - I tried.
Author:  joshnothing [ Fri Dec 17, 2021 3:07 pm ]
Post subject:  Re: Bridge reglue questions
Chris Pile wrote:
A warped plywood top will remain warped in my experience. The glue between the plies has moved and reset while under tension and heat. I never could get the hump to reverse itself, and believe me - I tried.

Thanks Chris. The top is solid cedar but this bendy bridge plate is laminated construction. Sounds like I’ll have to bite the bullet and get it out, I’ll report back.
Author:  Hesh [ Sat Dec 18, 2021 7:37 am ]
Post subject:  Re: Bridge reglue questions
Cordoba guitars often have cedar tops.

We tell people that the word Cordoba (even though this is not a Cordoba) means bridge reglue in Spanish.

:) Cedar tops and bridges seem to have shorter, less successful marriages so maybe builders with cedar should consider their prenups? Now watch me get blasted by all the folks who build with cedar. I built with it too and it does shed bridges seemingly more often and then it does it often is not pretty.
Author:  jfmckenna [ Tue Dec 21, 2021 11:31 am ]
Post subject:  Re: Bridge reglue questions
Wow yeah that is a terrible design there.

I've had luck clamping hot wet felted blocks, much like the Thompson but cheaper, to flatten out such things. I think I could dig up pics if you need it but basically take a wood block and laminate some wool felt to it (to hold moisture). Shape the block as needed. Boil up some water and soak the block in it a bit then insert it through the sound hole and clamp it for a couple weeks. Repeat as necessary. I have only ever done this on guitars built with HHG though so I'm not sure how well it would work here.
Author:  Chris Pile [ Tue Dec 21, 2021 11:49 am ]
Post subject:  Re: Bridge reglue questions
Intriguing technique.
Author:  joshnothing [ Wed Dec 22, 2021 6:19 am ]
Post subject:  Bridge reglue questions
Brief update, I decided I had nothing to lose by trying to press the area flat before moving onto bridge plate removal.

The patient has had one round with the belly reducer and seems to be somewhat responsive to treatment. Belly at the rear of bridge footprint is back down to a workable level. Still need to get some upward movement out of the section forward of that transducer rout. It really kicks down there - there’s barely 1/8” of bridge plate in front of the rout to support it and the whole area is weak.

Back into the clamps for round two.
Author:  joshnothing [ Fri Dec 31, 2021 12:41 am ]
Post subject:  Re: Bridge reglue questions
Brief conclusion - the second 24hr treatment got things flat enough for a glue up. Bridge was scraped and sanded to a good fit, bridge footprint cleared right to the edge to max out gluing surface. Bit of preheating of bridge components, some HHG and a clamp:

Image

A warning on the current iteration of SM bridge clamp - the rubber contact pads will eat into a finished bridge, or at least they did for the finish on this bridge. It’s a precatalysed satin nitro. So after 24hrs clamp time there was a few minutes of masking and touch-up before it was time for strings:

Image

Belly all but gone:
Image
Author:  Hesh [ Fri Dec 31, 2021 3:43 pm ]
Post subject:  Re: Bridge reglue questions
Looks great Josh, good going!
Author:  joshnothing [ Fri Dec 31, 2021 4:22 pm ]
Post subject:  Re: Bridge reglue questions
Thanks Hesh. Happy new year!
Author:  Chris Pile [ Fri Dec 31, 2021 4:39 pm ]
Post subject:  Re: Bridge reglue questions
Well done!
Author:  joshnothing [ Fri Dec 31, 2021 5:08 pm ]
Post subject:  Re: Bridge reglue questions
Thanks Chris!
Author:  SteveSmith [ Fri Dec 31, 2021 5:52 pm ]
Post subject:  Re: Bridge reglue questions
Nice, clean job.
Author:  Lou Thier [ Wed Feb 09, 2022 5:26 pm ]
Post subject:  Re: Bridge reglue questions
your experience is pretty much in line with mine! belly reduction does work! not a miracle cure all, but it does help! BTW nice job.
Author:  slightreturn [ Mon Apr 18, 2022 11:43 am ]
Post subject:  Re: Bridge reglue questions
This is an Elger 12 string I did about 6 years ago. Nasty, nasty hump. The top was severely deformed and there were loose braces inside as well. The real culprit though? The bridge plate. The before and after you see there is the old plywood plate that basically was doing nothing, vs. my new maple plate (with grain running parallel to the strings), which I epoxied in place with soundhole clamps and a caul *while the top was clamped flat on my Go-Bar deck as well*.

I had reglued the braces and the bridge originally, and it came back about 2 weeks later with the bridge coming off again and the top even worse than it was originally. At first I thought the brace reglue failed, but nope. And I glued on the bridge with 2 ton epoxy so that was also not the culprit. It was quite a shock to see a bridge I had epoxied on lifting off again, almost completely off the guitar. It only took a couple minutes and no heat to remove the bridge with a spatula.

After removing the bridge a second time (with only one small tearout, mind you!), I removed the plywood bridge plate and painstakingly made a new one out of maple. The original plate was very small and very thin, and by now it was in pieces, so I had to use popsicle sticks and scrap MDF inside the guitar to draft out a new plate that would fit tight against the bracing.

With the grain running parallel to the strings, I had the plate done. Here's the kicker: I clamped the top flat with Go-Bars and a caul on a Go-Bar deck. Then used soundhole clamps and a bridge plate caul to glue the new plate in.

I used 2 ton epoxy to glue the bridge plate in as well. Ballsy, yes, and that will never come off, but it was a plywood top *and* it had come back a second time after my initial bracing and bridge reglue. The reason was the bridge plate failed. I thought that the epoxy might do a better job of locking the top in place and keeping it flat under tension if I used it to glue the new, thicker, bigger bridge plate in while the top was pressed flat on the Go-Bar.

And it worked. Played beautifully, and even sounded great. The action came way, way down to something actually usable -- low for an acoustic with a nice high saddle. Reglued the bridge of course, cut a bone saddle for it, and all the usual setup work. Came out phenomenal actually. My heart sank into my stomach when it came back after the initial brace and bridge reglue and it looked totally destroyed -- but fortunately, as I said, the culprit ended up being the bridge plate.

Sometimes you gotta get creative with these things. I did one a few years later a similar way, but didn't clamp it on the Go-Bar deck when gluing the new plate in. Just glued in a new maple plate with soundhole clamps and a bridge plate caul.

It did not come out anywhere near as good. I really do think that epoxying the new bridge plate in place while clamping the top flat made a huge difference. Industrial level repairs for something so severely damaged is not unreasonable in my opinion. It's possible you could get away with hide glue, fish glue, Titebond, or the like. In my case, I didn't want the guitar coming back again and I wanted a permanent repair.

But who knows? It's possible the epoxy helped stabilize the top in place. I used Devcon 2 ton so it wasn't flexible like G-Flex epoxy or anything like that. Not sure if it made a difference but that was one of the most solid repairs I've done with one of the most dramatic differences. This thing was ready for the trash heap.

Anyway, because of this, when I see severely bellied tops, I think "bridge plate." And then I remember clamping the top flat on a Go-Bar deck while clamping the new plate in place. I really, really have no interest in doing such a repair again any time soon (or ever), but man, did that work.
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