First Bridge Re-Glue: Soundboard Tearout?

[Tony Lewis Skypilot Guitar Repair]

Ya…..You’ll find out that there are quite a few companies that glue over the finish and not just on cheaper guitars. They’re like surprise packages…. You can make a paste out of material similar to the top. You can use rasped flakes (chunky better then sanded) mixed with any of the Titebond wood glues to form the paste. Don’t be afraid to use higher heat on the bridge. They’re usually unfinished and you can heat the crap out of them. Experimenting on cheapies like you are is a great way to find the limits. I’ve read up on melting temps of PVA glues and most soften at about 150 degrees. But you need to account for heat loss from the bridge material (wood most of the time) which acts as a “heat sink” which takes away a fair amount of heat from getting to the joint. Of coarse we need to worry about the surrounding finish but by heating the bridge only, you are limiting the heat to that area. And if you heat the spatula you’re using, you can limit the heat to the under bridge area by bending the spatula away from the top. I’m finding that you can heat the bare wood up to close to 300 degrees works well. Same with the spatula. Getting that glue joint at least to 150 is the key. Also, wood grain direction is a consideration that we often forget about. Most guitars have book matched tops which means the grain on the two pieces of wood can be going in different directions. You want your spatula to be going with the grain of the wood to limit tear out. Picture the hair on a cat. You go against the lay of the hair and the cat gets fussy. If you pet the cat in the direction of the lay of the hair, the cat purr’s. It can be tricky to find the lay of the grain on the top. You can look in the sound hole and usually can see the lay of the grain and extrapolate that to what’s happening under the bridge, fingerboard extension, etc. when using the spatula. Oh, forgot to mention that you also want to scribe the finish with some sort of knife, around the bridge, fingerboard extension etc. before removal so you don’t get finish/wood tear out as well. Hope this is useful info!

[Adan Akerman Akerworks Fine Instruments]

“Like surprise packages” 😂

That is a ton of useful info, thank you so much. You’re right, I was being very tentative about heat. Having a thermometer and a target temperature like that will be a big help (on that next crummy Facebook guitar over in the corner).

I didn’t previously appreciate the cat-fur-like aspect of the top’s grain. That could explain the part of the perimeter where the spatula just slid right between layers of the top, kind of a sick feeling when that happened.

Somewhere I’d seen Beau Hannam’s recommendation to always use a fresh scalpel blade and score around the bridge before removal, as you’re describing, but I was probably being a bit tentative with that too.

Thanks again, Tony!

[Ian Davlin The Looth Group]

Once you get the finish off, unless I’m mistake, that’s not going to be a tremendous amount of gap filling.

Here’s the rub on patches on the top under the bridge. You already encountered the issue when taking the bridge off. That wood under the bridge has a ton of run out. Well, if you glue something to the top there, that’s going to have run out too. The bridge staying on is the function of two critical things. One, the glue joint. Two that the bridge is glued to top material with continuous wood fibers.

Unless it’s more extreme than what I’m looking at, I’d glue this bridge down with either hide glue or epoxy (good epoxy like west systems). I’d also glue the remaining runout down first, before gluing the bridge on.

Id use epoxy if the gap was extreme, hide glue if it was subtle. I also would not use any cold hide glue if you use hide glue.

[Adan Akerman Akerworks Fine Instruments]

Thanks, Ian! Missed your note somehow. That Frank Ford article is great, such a clear explainer.

My rasped-spruce + Titebond wood paste filler is in place and has been curing a few days. I’ll sand it all flush soon. But from what I’m reading I think that filler choice will prevent me from using epoxy, right? The internet suggests PVA is effectively a release agent for epoxy resins? Or will the high spruce content of my paste make it work ok?

I’m glad it’s a learner guitar, this would be stressful if it were fancier!

I hired my eight-year-old to make the spruce raspings, not sure if we need to take that into account here. Not impossible there’s some booger content in there.

[Ian Davlin The Looth Group]

I’m not sure about the spruce slurry. It’s just that titebond to titebond joints are not great and destined for failure. If your titebond slurry has saturated your spruce, I’m fairly certain that it isn’t providing anything other than gap filling with very little bonding.

At this point, it would, IMO, be better just to glue in some spruce, even with all the run out stuff I mentioned before as long as there was fresh, unsaturated spruce to glue the bridge to.

In general, this is why it’s just a great idea to use hide glue. Do-overs don’t require excavating old glue etc and if you glue in patches for gaps, those can be redone as well.

[Ian Davlin The Looth Group]

Also, here is Frank Ford’s great old article on runout.

https://www.lutherie.net/frankford.runout.html

lutherie.net

Runout and vertical grain

Runout and vertical grain