Failed bridge repair what did I do wrong?

[cyclistbrian]

A couple worker has a solid cedar topped Alvarez which he'd left in his car and other hot spots. The bridge popped off. He'd attempted a repair but the bridge mostly lifted off again. This time it was a mess of stringy dried glue. I figured how hard can it be to do it right and I gave it a shot. I was wrong.

For reference I've worked in the trades, am an electrician by degree and training, a fair plumber, carpenter, and Stone mason. You get the idea. I'm handy and what's more, I have a fully equipped shop. This should have been a slam dunk.

I heated off the bridge. I sanded the bottom of that bridge until it was clean. Same for the top. There were some splinters of wood gone but overall intact. I filled the missing splinters with slivers of wood with superglue. The top was very hard to get clean of old glue. The glue Alvarez uses is hard as rock. One clue to the subsequent failure is it appears Alvarez uses some sort of epoxy perhaps incompatible with carpenters glue.

An additional detail. The bridge is ebony black but I wouldn't expect actual ebony on a lower spec Alvarez. I suspect died Rosewood but who knows? It sanded like wood. It produced sawdust. Alvarez appears to use "Tech Wood" on some cheaper models now but this guitar is fairly old.

When I was satisfied with my sanding I opened a fresh new bottle of Elmers carpenters glue. I applied it fairly thick. A bit of unorthodox, I secured the bridge with cap screws and nuts and washers through two string holes to keep it from moving. I clamped it tight with four bridge clamps. I carefully wiped away squeeze out and left the guitar in my dry climate controlled shop. It sat for three or four days.

I strung it up and it looked like success. Sounded good too. Perfect intonation was a sign I'd gotten everything lined up. Darned if after a couple of days that bridge was lifting slightly.

Cedar oils in the top? Elmer's not adhering to embedded epoxy? I welcome thoughts.

[Fathand]

You don't mention fitting the bottom of the bridge to the curvature of the top?

No doubt it was done at the factory but after all the sanding/glue removal it may have changed?
Carpenters glue is not a very good gap filler if there was a gap(s).

Usually sandpaper facing up on the top and sand the bottom of bridge to fit.

[BradHall]

Without a clean wood surface on the soundboard Elmers would not be my choice.
Profile the bottom of the bridge as suggested.

[TheGITM]

Caveat: I do not build guitars.

I was curious if you've checked the cross bracing on the sound board between the bridge plate and the sound hole? That cross brace should allow you to bring the strings to tension and prevent any bowing in the sound board. But I was thinking if the bracing has loosened then when you apply the string tension... if the sound board starts to bow, that bridge would almost certainly start to separate.

Just a thought from a non-builder.

I have only had one bridge separate and that was on my D55. I repaired it using Titebond and it has held fine.

[runamuck]

Elmers would not be my first choice - Titebond 1 or hide glue would. But with a tight clean joint it should have held.

I can only guess that you haven't gotten the surfaces clean down to bare wood yet. You mentioned epoxy having been used. If there is a significant amount of that left that's not good because the glues I just mentioned won't adhere well to it.

[Talldad]

Glue choice is unlikely to be the cause after such a short amount of time.

If the bridge is still attached check for excessive curvature of the sound board behind or infront of the bridge. As stated previously if the braces are inadequate you’re going to have to address that first.

If the board is sanded clean then it’s good enough for the glue. Putting a piece of sandpaper on the guitar top and rubbing the bridge over the top to get the curvature off the soundboard imprinted onto the bottom of the bridge is a very good idea. It’s laborious as hell, you can’t press too Hard Either as you change the shape of the soundboard.

Is the bridge particularly oily? If so clean it off with spirit before gluing.

[ssstewart]

if it was profiled and contoured to fit the top should of worked.

however, the best glue for anything guitar related that is under extra pressure (or anything else for that matter imo) - fractured neck/headstock or ill fitting bridge that isnt contourable is...

Mitrebond- this glue was recommended to me by an old cabinet maker a few years ago for the task of a busted headstock on a restoration i bought. this crap is beyond exceptional...but you got to be quick when fitting the joint. better than an epoxy or 2 part compound- basically its a ridiculously strong glue by itself that once you apply it to one side you spray the 2nd can (aerosol activator and fuser) to the other side that you are mating with. once you join the two peices..it aint coming apart..ever (within reason) even if the two pieces dont mate perfectly, but are close enough for government work and especially good when mating 2 different types of materials ( wood to ceramic/ granite to wood/ bone to ivory..you get the idea) ive had a joint break in a different spot entirely and not on the mitrebonded part when stress testing hardwoods. highly recommend it..just dont expect to be be able to remove the pieces again

[runamuck]

Originally Posted by ssstewart

if it was profiled and contoured to fit the top should of worked.

however, the best glue for anything guitar related that is under extra pressure (or anything else for that matter imo) - fractured neck/headstock or ill fitting bridge that isnt contourable is...

Mitrebond- ....)

How is this different from cyanoacrylate?

[cyclistbrian]

Thanks everyone. It sounds like I may have lost some of the contour and should have paid more attention to that along with choosing glue.

[ssstewart]

Originally Posted by runamuck

How is this different from cyanoacrylate?

much better than cyanoacrylate i find, CA, superglues etc dont compare for me, as it is a modified cyanoacrylate It does not
contain solvents so really no shrinkage. its more stable and viscous.
used by itself (just the glue part) its similar to CA yet not the same and the activator addition really helps with the bonding (it seems to neutralize the mating surface) doesn't get as brittle and deteriorate as much. more expensive though..but lasts a long time stored unused. my experience anyway with it.

[runamuck]

Originally Posted by ssstewart

much better than cyanoacrylate i find, CA, superglues etc dont compare for me, as it is a modified cyanoacrylate It does not
contain solvents so really no shrinkage. its more stable and viscous.
used by itself (just the glue part) its similar to CA yet not the same and the activator addition really helps with the bonding (it seems to neutralize the mating surface) doesn't get as brittle and deteriorate as much. more expensive though..but lasts a long time stored unused. my experience anyway with it.

Thank you.

[redir]

Depends on the radius of the top but for shallow radius guitars I simply glue the bridge on flat. If it's a big radius then I contour the bottom.

Did you have squeeze out all around the perimeter?

A good tool for prepping the soundboard is a 1in chisel with the sharpened edge 'bead' rolled back towards the flat side. IOW sharpen the chisel normally but on the last few strokes don't hone the back side leaving the edge to point that way.

You need to clean ALL the old glue off. Don't worry about a few small splinters.

[nickv6]

"however, the best glue for anything guitar related that is under extra pressure (or anything else for that matter imo) - fractured neck/headstock or ill fitting bridge that isnt contourable is..."
Just to say that in my life repairing pianos..ca type glues, superglues etc. work well for joints in compression but are poor in tension joints.
Maybe the mitre type overcomes this weakness?
Nick