Re-gluing a bridge

[Jack Lord]

I'm a hobbyist builder and I'm re-gluing a bridge on a 12 string guitar.  I know the basics and have done it before: scrape the surfaces clean, use hot hide glue, etc.  However, when I removed the old bridge there was a lot of damage to the spruce top under the bridge.  It seems that I must route out the damage and inset an inlay.  Does this sound like a good idea? What else could I do?  The problem in the first place was that the bridge was glued over a large perimeter of finish.  I will rectify this by making it full contact all the way around.  Thanks for your help.

[nathan clark]

When I encounter something this bad:

I use the same species of wood for the patches as that of the top and maintain the same grain orientation and run-out as the top.

I try to keep the glue joints away from the perimeter of the bridge's foot-print so the bridge itself helps span the problem area.

I try to keep the cross grain joints running as close to the grain as possible as the cross-grain joints are the weakest part of this sort of repair.

I keep the glue joints away from the pin holes because that's the top's folding point.

I avoid running cross grain joints. I break them up if I can by putting them at different latitudes and different angles. I think of this like avoiding running joints in a dry stacked stone wall.

If it's possible, I taper the thickness of the patch and mortise with the grain so it's thickest in the center, then feathers out to nothing.

I treat the patches like inlays. I lay them out, cut them out, then cut the mortises in the top and I'll radius the bottom of the mortise if possible.

I glue the patches in with hide glue, then plane chisel and scrape them flush with the surrounding glue joint.

I'll upload a photo of a rather extreme example of how I patched some previous damage I encountered during a bridge re-reglue.

[Jack Lord]

Thanks for the info Nathan, the picture really helped.

[Hesh Breakstone]

A couple of things come to mind for me when reading this thread.

First let's discuss how to remove the bridge because although Jack already has his bridge off what we share here may help the next person avoid top damage all together.  What better way to address top damage than avoiding it all together...

There are usually lots of ways to do most anything Lutherie related but I can share with folks how we have been removing bridges lately.  I say lately because our methods change and morph in time always looking for improvement.

Step one for me when I have to remove a bridge is reading the top...  All I am looking for is the direction of the runout, if any... so that when using my spatulas after heating the bridge I can attack the task from directions that will not pull up top fibers and moreover lay top fibers sticking to the bridge bottom back down.

To read the top I typically do so from the neck side although it does not matter.  This is simply how my bench is situated giving me my best view from the neck side.  I sight down the neck looking at the two halves of the top and what I am looking for is which side is darker.

Just like striping one's lawn runout will appear darker when viewed from the direction that the runout is going toward.  So if by chance the left side of the top, when viewed from the neck end of the guitar, looks darker AND the right side of the top looks lighter in shade we now know that this top has a) some runout... and b) it's direction on the left side (again when viewed from the neck end of the guitar) is toward the neck end of the guitar.

With this information I approach the bridge removal process with the spatulas inserted first on the back edge of the bridge on the left side.  This lets us lay loose fibers back down using runout as not only a reality but something that can compliment our efforts to not damage the top at all.  Likewise approaching the bridge removal with the spatulas from the front of the bridge on the right side and we are again laying down loose fibers and not tearing them up with our efforts.

The second thing that comes to mind is the issue avoiding lifting fibers by being sure that we get the bridge hot enough so that the glue is soft and gives way.  Lots of tops get damaged in the bridge removal process because the bridge has not been heated enough to soften and release the glue.

Regardless of what method that one uses to heat the bridge, and there are likely lots of ways to do this too...., the goal is to get the bridge hot enough to soften the glue and not smoke/bubble the finish in the process....

Currently we are using heat lamps with shields that we make and keep for every conceivable bridge shape (fret board extensions too) that we might encounter.  After heating the bridge (with constant supervision when using a heat lamp on a guitar...) for a while, one gets a feel for just how long, feeling the level of glue resistance with the spatula will give you some feedback.  What we want to see/feel is the glue is softer now and will give way so some pressure, not much, from the spatulas.  Once the glue is soft enough you will see what I am describing here, the glue gives way and the spatulas require only constant but minimal pressure to make headway under the bridge.

Approaching the direction of the spatulas with the runout direction in mind and it's possible to remove a bridge with minimal to no tearout.

I wanted to bring up how we reglue bridges too but will start another post for that one since I am such a wind bag....

[Tim Mace]

whew.......

[Hesh Breakstone]

I can relate... ;)

[Jack Lord]

Thanks for the pointers Hesh.  I was definately too aggressive in removing the bridge.  Please continue on re-gluing the bridge.

[Hesh Breakstone]

I thought that Nathan covered repairing top damage very well and Frank has a toot on here that maybe someone can post a link to where he replaces top damage with a scoop of new material and the results are excellent.

So the other thing that I wanted to discuss is how bridges are glued on in the first place...

As we all know from experience f*ctories basically do a lousy job of installing bridges.  There are a few exceptions but most of the bridges that I remove still have a large percentage of the bridge foot print where the finish has not been cut back preventing any wood-to-wood contact where the finish remains.  There is also the issue of the bridge having to "span" the ledge where the finish is and is not....  Depending on finish thickness some glues that are not good at gap filling such as hot hide glue will not have the wood-to-wood contact that HHG greatly benefits from.

Some of the examples of removed bridges that I have worked with show such a sloppy and incomplete job of finish removal under the bridge that nearly 40% of the bridge's foot print is not glued to the top at all.  And it does not seem to matter which of the major f*ctories either, I've seen poor bridge glue-ups from some of the most notable...

Jack the OP is going to improve the wood-to-wood gluing surface available by cutting the finish all of the way back.  That's one way to do it but this method relies on making a perfectly sized and shaped area where the finish is removed or it will show around the bridge if one is off by even a hair.

Not cutting back the finish as the stinkin f*ctories do or not cutting enough of the finish back is problematic in so much as it's a weaker joint then necessary especially along the back edge of the bridge where the heavy lifting is done by the fit and glue joint.

A third possibility is to route a very fine ledge around the bridge bottom, I am speaking of a ledge that is approx. .003" high from the bridge bottom and .003 - .005" inward from the bridge perimeter.  We have a jig that does this and does it nicely too.  The last step is to remove finish from the top just inside the bridge foot print so that the ledge around the bridge bottom fits into this "wellt" in the guitar top's finish.  This method uses all of the bridge foot print less that .003 - .005" inset and nixes the idea of having the bridge bottom attempt to span where the finish ends and bare wood begins.

As such the bridge glue joint now uses nearly the entire bridge perimeter for direct wood-to-wood contact increasing the area of adhesion including on the back edge which is a good thing...

Additional benefits are that the bridge when properly positioned during the reglue "snaps" into the "well" created by the decent fit of the bridge bottom to the well in the guitar top making repositioning the bridge, quickly if using hide, fast and easy.

Lastly since we have stayed inside the bridge perimeter while cutting back finish, if only by .003 - .005" it looks better in so much as the areas where the finish has been cut back are all under the bridge and not in view.

In my experience the ledge is so very shallow that even though some may claim that the ledge makes it more difficult to get a spatula under there in the future this has not been my experience.  Besides it's highly likely that if this bridge ever has to be reglued again it would be because it has started to lift providing access for the spatulas anyway taking into account my previous long winded post mentioning using top runout to our advantage.

Not addressed here is the method of shocking bridges off which we only use on cheap, imports that may have the bridge glued on with AMG (Asian Mystery Glue) or have the bridge glued directly to a poly finish.  That's another topic....

[Jack Lord]

Thanks again Hesh.  The bridge I removed overlaid the finish by a good 1/4" on the back of the bridge, right where you need all the adhesion you can get.  By the way, this is a Sigma guitar made by C.F. Martin in Korea.

[nathan clark]

It's best to score the finish around the bridge before removal.  Failing that, it also works to clamp the bridge down located as precisely as possible on the original foot print then score the finish.  OP, don't feel bad; bridge reglues on nicer guitars tend to be much easier than your current experience.

FWIW, here is my non-basket case bridge reglue routine:

http://fingerlakesguitarrepair.com/martin-bridge-reglue/ 

[nathan clark]

Hesh, are you willing to share some pics of your ledge routing jig?

[Hesh Breakstone]

Sure thing Nathan.  I can't find my pics of our jig at the moment but here is a pic of Terry Kennedy's which is very similar to ours.

[Hesh Breakstone]

It's a Drem*l mounted in a Stew-Mac router base with the addition of some additional hardware to end up as you see it.  Typically the "tables" are radiused a bit, 25ish will do and of course the depth and inset of the rabbit is completely adjustable.

Regarding your method, Nathan where you indicate it's best to score the finish around the bridge prior to removal this most certainly works well for you and if one is looking to cut the finish back under the entire bridge.  That's one way to do it and I used to do this with my own guitars that I build.

I moved away from this method in favor of the rabbited bridge perimeter because no matter how careful one is this method leaves zero room for slips and mess-ups...  I had good success with it but I never liked being able to see the glue line even if only in very limited areas and looking specifically for it.  It's an excellent way to go for improving the amount of wood-to-wood contact over what the f*ctories do not cutting back the finish closer to the bridge perimeter I would agree though.

This idea of rabbiting bridges is not new and although I can't remember who introduced us on another forum to the idea years ago I do know that Collings has been doing it this way for a long time.

What you end up with is a very neat and tidy bridge glue-up with the finish nicely visually extending under the bridge and no glue line or sloppy scoring showing at all.  The trade off is the .003 - .005ish of bridge perimeter that we lose for wood-to-wood contact but again compared to how the guitar was originally manufactured it's a huge improvement in gluing surface area.

Both methods benefit from the advantages of positioning, rapidly and correctly... the bridge in a well in the finish and the risk of the thing sliding around is minimized as well if properly positioned and the well matches the bridge rabbited perimeter.

I also used to use a heat gun to heat up my bridges too prior to glue-up but these days lazy me likes the microwave for 15 - 18 seconds better.  Besides the microwave is easier to use to heat up my pastrami on rye with Swiss sandwich too... :)

I liked your toot as well Nathan and wanted to make a few other suggestions of other ways to do things too if I may please.

Instead of using a blunt stick or object to clean up the jelled HHG I pre-position and clamp the bridge and surround with masking tape where the straight edges may be.  What this gives me is an even better defined well for the rapid positioning of the bridge AND 2 minutes after the clamps are in place removing the tape removes roughly 90% or so of the glue in one felled swoop.  Since the finish is still tucked under the bridge even if it's only .003 - .005"ish the risk of lifting loose finish with tape removal is greatly reduced.  For the rest of the clean-up I wet a quality and soft paper towel (Bounty Select-a-size....) in the HHG water bath and push little damp pieces of the towel around the bridge/top junction with a slightly dulled tooth pick that may have also come with my pastrami sandwich....  You want the pieces of paper towel damp and never wet so as to not have water seep under the bridge...

Lastly for now although HHG is truly wonderful stuff with examples of Egyptian furniture made with HHG and 2,000 years old surviving in museums HHG is not all that forgiving if one cuts corners.

Preheating parts if you can't get the clamps in place in say 15 - 20 seconds is advisable and helpful as are dry runs and not in the gastronomical sense...  Any method that we may develop to preposition clamps, cauls, etc when using HHG can save precious time.  If the glue starts to jell before all things are as we want them it's compromised and 15 - 20 seconds is not a very forgiving amount of time.

Again there are lots of ways to do the same task and likely more ways than any of us can think up too.  Our methods morph and change regularly in our own quest to do great work and provide real value.  I look back at some of the toots posted on my own web site and in most cases now what I once did I no longer do.  And perhaps this is the attraction of Lutherie to me in so much as it's dynamic at least in the learning and a lifetime is not enough to even scratch the surface of all there is to know and learn.