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Not enough room in the title section to add names of other repair experts like David Collins, John Hall, Hesh, David Newton and many others, but I have a question about regluing a bridge on a Martin guitar.

I left my 2003 Martin OM18GE in my car one summer day (stupid me) and of course the bridge began to lift in the back. I have succesfully removed the bridge, but now I am faced with glueing the bridge back on. I think the bridge is salvagable, but the problem is that the surface of the top is not flat, and neither is the underside surface of the bridge.

When I lay a straight edge along the top, going along the grain, there is a slight hump where the string holes are, and also, laying a straight edge across the grain where the bridge used to be shows that the top has cross grain convex radius, or a hump accross the grain. Conversly, the bridge is not flat, and it has a radius across it's length, and a hump where the string holes are along it's width.

My question is: should I try and flatten the bridge and the top out a bit? Or is there a way to match the odd topography of the bridge footprint on the top to the shape of the bridge itself? How do you folks glue new bridges on old tops, it seems like it's hard to get a proper gluing surface on an old top. Especially for hot hide glue which demands a gap free joint.

Please note that this guitar was one of the Martins that had excess lacquer under the bridge, which I suppose was done for ease of bridge glue ups, and for the fact that it leaves a clean look around the bridge footprint. But, it also left the bridge poorly glued to the underlying wood. I have already cleaned out this excess lacquer from the bridge foot print, so that is not the issue here.

Any advice would be much appreciated.

Best,

Jonas Baker

Photos would help. Closeups would be VG.

One method would be to chalk fit the Bridge to the top, scraping the high points until you reach an exacting fit. The method is a little slow but perhaps the least invasive, as all the hollowing is done to the bridge. In other words you are making the bridge fit the top, not the top fit the bridge (or some sort of averaging). It's a common technique when restoring old and expensive instruments, whereby you wish to preserve as much as the original as possible.

Hi Jonas - It's frequently the case that when bridges lift and have to be removed and glued back on there will be top distortion and the bridge itself will distort as well with a common sight being the bottom of the wings curl upward.

Unfortunately it's also pretty common that most manufacturers don't clear enough finish under the bridge especially on the trailing edge where most lifting starts.

Step one: Inspect the bridge plate carefully for worn pin holes, a crack or even a fold developing along the pin holes. A common reason for bridges lifting is bridge plate deformation due to wear, cracks, enlarged holes, etc. The goal if bridge plate issues contributed to the lifting bridge is to fix the root cause before regluing the bridge. And slotted pins can be evil but that's another rant...

A 2003 is new enough that this may not be an issue but it is a common issue on slightly older and much older guitars.

It's also common for bridges and tops not be flat, far from it usually.... And of course many of us intend to have a top radius too....

Step two: we clean up the bridge bottom on a belt sander flattening it out and removing old glue and any lifted top fibers. It's important to have a belt sander with a decent, flat platen and it's also useful to know your platen so you know where flat results can be obtained.

Step three: We take this, regluing a bridge a bit further than most shops in that we want as much wood-to-wood surface as possible so we will trace the existing bridge moving it very slightly inset from the former bridge perimeter. To be more clear instead of clearing finish to the edge of the bridge we hold it in place and then move it about .005" into it's own foot print and then score the finish. What results is that the finish will be cleared eventually completely under the bridge except for .005" from it's perimeter. What results is no visible glue line and use of around 95% or more of the bridge's foot print for gluing area.

We use very sharp chisels on the top to remove the old glue, and crap including some of the finish since we are expanding the gluing surface. The goal is all old glue removed from both the bridge bottom and the top of the guitar.

Step Four: Now comes the fitting of the bridge to the top, the part that I believe you will be most interested in. There are lots of ways to do this, some slow and laborious and some very quick but require some skill and experience to get it right. Again that belt sander can be your friend to sand the bridge bottom to the top shape. Typically the top is not flattened and used as is unless the distortion is major or the result of damage. So concentrate on matching the bridge shape to the top.

There is some disagreement in the trade about how well bridges need to fit. Some folks will clamp the heck out of them and make them fit, others want minimal clamping pressure needed to seat the bridge with full wood-to-wood contact. I'm in the minimal clamping pressure required camp at present (things change...) and have had great luck shaping to fit and then clamping the heck out of it...

A single edged razor blade used as a scraper can help refine the bridge bottom shape as well and is also a good idea too once you have it all shaped to fit to lightly scrape the bridge bottom just prior to gluing for more "joint energy..."

Step Five: Once you like the bridge fit and maybe it only needs light finger pressure to snug the wings and everywhere else down it's time to get ready to glue it on.

We position the bridge in it's exact desired location and clamp it in place dry. Once in place we run masking tape around the bridge front and sides to make a "well" of sorts that the bridge can be slapped into place quickly for the glue up. These days I am doubling the tape thickness to have a deeper well for easier positioning and less chance of clamping the bridge over some tape...

Once the bridge is fitting the top, the top and bridge are prepared and cleaned up for the glue-up and you have positioned the bridge and made your tape well it's time to glue the sucker on.

With HHG you have very little time, I use 15 seconds or less unless things are preheated. Different shops have many different ways of doing the glue-up and the method that we use permits the clamps to be prepositioned requiring only 10 seconds or less to have clamps in place and snugged down. I don't have any pics of this method and it's not my method either to share so let's go with something else.

Lately I have been preheating my bridges in a 600 watt microwave for 15 seconds. It's almost too hot to handle and greatly extends open time when you preheat the bridge.

Have everything ready and at hand. Also do a dry run or two timing yourself so that you are familiar with what to expect.

Have your clamp and caul at hand and adjusted so the clamp fits through the sound hole and the bridge caul does not have the adjustable wing clamps deployed yet.

You will also need a caul inside the guitar that fits over the bridge plate. Some folks use inside cauls that span the X and bear outside the X too. Depending on what kind of bridge design and manufacturer this is not needed very often

Cover the caul with wax paper so that you don't end up with a 3/4" thick bridge plate....

The bridge plate caul can be prepositioned and taped under the guitar top with masking tape saving you a step during the short open time of the glue.

Once it's all in place, dry runs have been done with no issues go for it! Slather some 145 degree HHG on the guitar top (or bridge or both but it's not necessary...) and slap that preheated bridge down into the well that you made out of tape. Manipulate with your fingers until you feel it snap into the well, position the bridge caul on top of the bridge, deploy your clamp and sung it all down.

Some of the commercial bridge cauls with adjustable wings can reduce the central clamping force if the wings are snugged down too much so perhaps get a feel for this too prior to using the glue.

Once the clamp or clamps (you can use more than one but it's typically not necessary) are in place all that is left is to wait about 2 minutes before removing the tape and cleaning up the HHG squeeze-out with paper towel and some hot water from you glue pot. A toothpick is helpful for right up against the bridge.

Leave the guitar clamped up for 24 hours. Clamps can be removed sooner but the bridge should not be stressed sooner than 24 hours.

What's left is to remove the clamps and cauls the next day, clean up any glue smears that you didn't notice the first day (hot water and paper towel). Next you want to drill the pin holes with a 3/16" drill and ream them for the pins with a 5 degree reamer.

Sometimes the saddle slot can distort a bit after the trauma of a removed and reinstalled bridge so the saddle may need to be fitted so that it's not too snug or too loose. Saddle fit is very important if the guitar has a UST.

String her up and you should be good to go!

Hope this helps!

Jonas.
As you will find there are a number of different ways to do a single job and you will find everyone has their own way of doing them.

First I am not an authorized Martin repair guy but I have repaired many many bridges and have yet to have one come back that failed.

First in my humble opinion I would never take a belt sander to a bridge that has pulled loose from any guitar. The reason your bridge and your top are not flat is because it was built that way. The top was built with an arch to it and the bridge was made with the same arch. Over time the top will develop more of an arch or "belly" from the tension of the strings. So you should not flatten anything.

As has been said you need to clean all the glue and debris off of the bottom of the bridge. This can best be done by heating it up and scraping with a sharp chisel or as I use a flat chisel edge X-acto blade. The top can be scraped clean with probably just the blade since most of the glue probably was left on the bridge as the top wood is much softer than the bridge. But if there is any glue there make sure it is all cleaned off.

I personally don't scrape the finish off to to edge of the bridge but I put a slight bevel on the bottom edges of the bridge so it just clears that little lip of finish. I try to keep the tolerances very tight and keep checking as I scrape the bevel. The finish shouldn't go any more than 1/32 of an inch under the edge of the bridge and usually I will keep it about 1/64". Yours is most likely a lot more than this so you will need to place your bridge down and put the 2 end pins in and make sure it is lined up right were it was originally. Then lay masking take all the way around it being careful to mask right up to the edge of the bridge. Take the bridge off and then scrape the old finish off leving that 1/64th of an inch or so. I never score a finish because you run the risk of scoring right into the wood and is is possible that your bridge will tear off right at those score lines only this time it will pull a layer of wood with it...

Now that you have everything clean and ready, You will need 80 grit stick-it paper and some 120 stick-it. Cut a piece 80 grit paper about 1 inch larger than your bridge and stick it to your jeans or pant leg and pull it off a few times to take most of the sticky off of it. Then rap it on its side on your bench to get any loose grit off. Now stick it on your guitar over where the bridge goes. What you are going to do is lay the bridge on the paper and rub it back and forth not going past the edge of the paper. The idea is to not put much pressure on the bridge being sure not to deform the top of the guitar. Lightly sand it and keep looking at the back of the bride to make sure it has sanding marks across the entire surface. When you get close you can use a pencil and just put some hash marks on the back of the bridge fro edge to edge. I use a white colored pencil so it shows up better but a regular one will do. Repeat the sanding process until all the marks are gone. Then switch to the 120 grit and make sure you do the jeans thing to remove some of the sticky. Do not use much pressure when sanding with either grit since you will be putting a different radius on the bottom of the bridge.

After it is all sanded at no pencil marks remain, use your chisel or flat X-acto blade and scrape the back of the bridge lightly to remove any sanding marks and you are left with a freshly scraped finish. The only thing left to do is to do a final scrape on the top and then I usually put 3 or 4 layers of tape directly over the taped off section you did to locate the exact position of the bridge. This will give you a small step so when you are ready to glue your bridge it will fit right in the exact correct spot.

I personally use hot hide glue but you can choose to just use Titebond Original if you are not familiar with HHG.

I am assuming you know how to make the cauls for under and over your bridge for clamping so I won't go over that but this is how I do my bridges. By putting that slight bevel under the edge of the bridge you can keep a really clean look and you won't see any cut lines where the finish has been scored off.

As I said, People do things very differently so there are a few ways to do what you want. You just need to find the one that works for you.

Hope this helps,

Cheers,
Bob

Make a new bridge with the desired radius (possibly use the old one).

Make a nice beefy inside caul (with or without a radius. Not sure how Martin does it).

Align with the pin holes, and clamp it with at least 3 clamps and show it who's boss.

_________________
Pat

Thank you everyone for your replies. I will try and take pictures tomorrow, but I've been out of the shop all weekend.

It does seem like there are a lot of ways to do this. I do feel that Martin did radius the bridge prior to installation, and that the bridge was and top was malformed from the string tension, and probably also from the bridge lifting as well. Because of this, I do like the idea of trying to match the radius of the top top to the radius of the bridge, and because of the tops slightly imprefect radius along and accross the grain, I think I will try the chalk method to match the bridge to the top. I have used this method in the past with dovetails, so hopefully it will work well here, but I do find that it is a bit of a messy method.

Rusrob, I wish I had read your method for removing the excess finish under the bridge. In this case it was closer to 3/32 of extra finish under each side of the bridge. I did end up very carefully scoring the bridge footprint, being extra careful not to cut into the top at all, but It seems that your method would have saved me some trouble.

Once again, I appreciate everyones replies. If anyone has any more advice on bridge regluing on older guitar, I would appreciate the input as well.

Best,

Jonas

Belt sanders are used by countless repair shops to clean-up the bottom of the bridge, old glue, very small lifted fibers, etc. Additionally belt sanders are also frequently used to do the basic shaping of a bridge bottom with refinements being done via scraping or other methods. It's a great tool and any shop that does any real volume of repair work is likely to use a belt sander at least to clean up bridge bottoms.

Not all tops are built with domes.... and more than a million Gibsons, Taylors, and Martins have been built with flat bridges that have not been fitted to any dome even when a slight dome exists.... Instead manufacturers glue flat bottom bridges onto tops domed or not. Of course not everyone does this either but it's fair say that the production standard for acoustic guitars when considering who produces the majority of higher end instruments is to glue a flat bottomed bridge as is.... When an instrument is new too top deformation has not happened yet making fitting bridges more predictable.

Martin I know at least in the past had bridges of different heights, slightly different heights that they used to select as need be for the neck angles that they were attempting to match. But the real point here is that the fitting that many of us do, I do it too with my own creations, is not the standard of the industry by any means.

As I said I like to fit when necessary (it's not always necessary either...) and I am looking for minimal pressure, just fingertip pressure to properly seat the bridge before deploying glue, clamps, etc.

Fitting is fine to do, it's required as well when I can't get a good fit without fitting and minimal finger tip pressure but the notion that all bridges are fitted at the time of manufacture is incorrect - just ain't so....

Regarding scoring finish to a line that you want expanding the wood-to-wood surface. Here too countless Luthiers score finish and most of us have the skill and knowledge to know in advance that we are attempting to score the finish.... not the guitar top. Scoring the finish can be perfectly safe that is if one truly knows what they are doing. Additionally many older guitars may have flakey finishes that are just waiting for an opportunity to pop off a chunk that is NOT under the bridge foot print and would show. Short of scoring and carefully working with say a chisel, scraper, etc. or routing as some do too, carefully..., not scoring the finish is risky and can lead to having to do some finish work that should not have been necessary.

There is always.... ALWAYS that "do no harm" ethic in play too. Scoring a finish (not the top....) is cheap insurance against doing harm to another person's property and for us what we unaffectionately refer to as "scope creep..."

Again lots of ways to do these things and as always results matter.

The Great Hesh is back....

Man I am not sure how we got along without your grand knowledge to correct everyone that YOU think is wrong...

Really ???? Uncalled for !!

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Like Wud said, really uncalled for.

_________________
Steve Smith
"Music is what feelings sound like"

And, there are those of us who actually believe that it is desirable to clamp and glue a flat bottomed bridge to a radius top, because that bit of tension will help keep the bridge and bridge plate from deforming under the relentless pull of the strings.

_________________
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Frank Ford

FRETS.COM
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Absolutely! Bridges are not only one of the largest braces on a guitar top but one of the most important ones too.

Amazingly there often are rhymes and reasons for how things are and have been done. A great example of this is how many folks have tried as they will to improve on X-bracing (including me...) but at the end of the day how a traditional acoustic guitar is assembled, say ala Martin is a pretty good way to go all things considered.

Yea you are probably right.... I shouldn't have been thinking out loud.

Bob

As I said, I have done many bridge replacements and early on I didn't put a radius on my bridges and always had a problem with the ears not fitting or pulling the guitar top up after I took the clamps off (it ends up leaving little kinks in the top beside and to the back of the bridge). I started putting a radius on the bottom for that specific reason.

So you are saying that I was doing it right when I didn't know better?

I would be very curious if everyone puts flat bridges on radiused tops and I am really that dumb?

I am not meaning this is a sarcastic way... I really would like to know.

Bob

Hi Jonas -

Lots of things to consider, lots of ways to approach it. Most important in my view is good diagnosis before prognosis and prescription. You bring up concerns about the top condition - and it should have a nice shallow crown or radius. The hump or slight fold at the pin line though is what deserves the closest scrutiny here.

Causes can include bridge plate wear or splitting (not so likely on a guitar this new), or creep and flex over time. If a bridge pulls away gradually leaving the front half torquing the top where it is still attached, you will often find a slight hump here. Or if it suffered severe heat under tension this could indicate the bridge plate slipping just as joint of the bridge itself gave way. Neither is good, an appropriate response can often depend on the severity.

If it's a major fold, it may require more drastic measures such as removal and replacement of the bridge plate (a major job not to be taken lightly). If it's just a minor unnatural hump which can be pressed flat under moderate pressure, then it can be corrected to some degree with the bridge reglue.

Perfect fit zero stress joints are great, but the bridge is one of the major braces on a soundboard, and if well glued can support a bit of stress to flatten out the top. In other words, if the bulge at the pin line is modest, I would not attempt to fit the bridge perfectly to the malformed top, but rather shape the bridge to help flatten out the top back toward where it should be. The radius along its length should match the average radius on the soundboard (sometimes I'll even over radius this a hair), but by leaving the front to back flat or just a tiny hair concave you can use this joint to help flatten out the bridge area.

Definitely make sure you have a good wood to wood fit, and this approach may not be suitable if unusually heavy clamping is required to force it to conform. If it mates tightly with modest clamping pressure though, this method of flattening slightly warped tops has proven quite effective and reliable over the years.

_________________
Eschew obfuscation, espouse elucidation.

As to gluing a flat bridge to a radiused top, or an over-radiused bridge to a (relatively) flatter top, I do both on occasions. Choices can vary depending on the individual situations I'm trying to address - sometimes concerning abnormal bulge behind the bridge, or other cases where the more prominent issue is the top flattening or collapsed in front. In either case it's an approach engineered to address a specific case, and why each job can be different and require case-specific solutions.

_________________
Eschew obfuscation, espouse elucidation.

Thanks David. I appreciate you taking the time to explain it.

When ever I have run into a top that was deformed I address it at the bridge plate rather than the bridge. I never thought the bridge had enough area to do the job.

Bob

+1



I almost always use a flat bridge on Martins, unless the top is severely humped front to back. If that is so, the stress on the bridge from clamping the top flat can lead to cracking between the pin holes. In that case, it is better to flatten the top by heating the bridgeplate and clamping it flat, or by regluing or replacing the bridgeplate.



AFAIK, Martin has always used a flat bridge, and I do it on all my new builds. The slight radius of a Martin top (~50 foot) is not so severe that clamping it flat is a problem.
Gibsons are a different story.......



I don't use a belt sander, but I do see how it would make short work of cleaning off old glue and flattening the bottom of a bridge. But there is one problem. If the bridge wings are twisted from a partial bridge lift, sanding the bottom flat will cause the bridge wings to be thicker on the upper edge. In that case, it is better to heat the bridge and clamp it flat before surfacing it.
I use a card scraper to remove old glue from the bridge, and sand it with 60 grit. On HHG jobs, I like to score the bottom of the bridge with a toothing iron. I also preheat the bridges for gluing with HHG, using a small heat gun.

Instead of using tape to locate the bridge, I use two small 1/2" brads through the bottom of the saddle slot. I don't like to use tape on old, fragile finishes because it can lift some finish when it is pulled off. The brads are rubbed with wax, and removed once the glue dries.
I also rub wax on the gluing caul for the bridgeplate to keep it from sticking. Waxed paper works, but it is easier to grab an old candle and rub it on the caul.

_________________
John

Regarding tool use for cleaning and shaping the bottom, I would say the choice of tool is far secondary to the skill of the operator. I use a belt sander for the majority of this work, but I know the platen well and use intentional care with pressure and placement. I can easily shape all sorts of curves, concave or convex, simple or varied, heavy removal or skimming of thousandths here or there. The sander in our shop used to belong to Dan Erlewine in the 60's, and to his father before he used it, so it's old enough to have some nice curves worn at both ends of the platen. Learning these nuances and how to use them as features is key to intentional craftsmanship. Of course if you're not fluent with the sander and observant of the belt condition to adjust your approach accordingly, it can allow for some big mistakes as well.

It should be noted that any other method can be equally risky if the operator is not skilled with it. Scraping can easily result in ripples around the holes, or beveling away an edge slightly with a slip, and if you're not fluent in scraper use it can be quite easy to chase problems around and make them worse.

Even fitting with sandpaper over the top is not nearly as sure a method as many would assume. Whether it be from deflection or irregularities of the reference surface, or varied pressure on different areas throughout your sanding strokes, getting the bridge to exactly mirror the top you are sanding against can bring many more challenges than one may assume.

Of course a good craftsperson can get ideal results with any of these methods, and make it seem near effortless if they are experienced with the approach. The results and risk though are much more a factor of operator skill than which particular tool they choose to employ.

_________________
Eschew obfuscation, espouse elucidation.

Thanks guys for answering my question and for being kind enough to take the time.

First, I know this is off topic, but I would like to apologize for my comment to Hesh. It isn't something I should have posted and in my frustration I hit submit without re-reading his post, I read something into it that wasn't there. I would like to give an explanation why I am frustrated if I could. You may know I am in the middle of setting up my new shop and was ready to install the heater before I put the insulation in. I just found out that it is going to cost me almost $5000 dollars and I only have about half of that budgeted. Because my shop is torn up, I have had to refuse a number of jobs so I feel I am sinking here. I don't make a ton of money as it is and this setback isn't helping. I know that is still no excuse to make a post like I did and I am sorry. Hesh an I have past issues relating to the same sort of thing that caused me to make that post but I should have read his reply without an attitude.

So anyway, I apologize for not keeping with the OLF standard and hope you can understand. As for the help in answering my question about putting flat bridges on, I appreciate your input and explaining it to me. For a number of years now I have always matched the bridge to the radius on tops and have addressed deformed tops at the bridge plate. This gives me something new to think about.

I am not too old to still learn something so thanks,

Bob

Good on you Bob Been there done that have the scars .

I have applied them flat everytime , because I thought that was the way its supposed to be done Never even occurred to me to radius one .

_________________
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Thanks from me too Bob. I understand and all is forgotten if that's what you want.

If there is anything that we can do to help you until your shop is back up please let us know? We are not too far away and would be happy to help a fellow OLFer.

Great post John!

One of the things that caught my eye though is that you are into keying (scoring) with a toothing iron prior to HHG use. There are a few threads here on the OLF where there is lots of disagreement on the need for using a toothing iron with HHG and other non-gap filling glues.

Might be interesting to hash this out again now that your a member too so that you can tell us the benefits as you see it. In the past when using a toothing iron was not recommended there was no one here to voice the other point of view. I like to think that I have an open mind (I also like to think that I have a mind.... ) but since I am pretty good at being wrong at times this would be an interesting discussion to me.

Along that line recently it was asserted rather dramatically I will add that HHG is an excellent gap filler. This is not my understanding but again maybe I'm wrong. This too would be an interesting conversation to have, some assigned testing, etc. It's also related to the idea of using a toothing iron with HHG since gaps obviously result or may result where the glue gets thicker.

Thanks

put me down as a flat bridger.

lets make one point very clear , the top is always in flux. it will rise and fall with RH changes. I have seen bridges that were domed and as the top dried out pop off. Martin has been going flat since 1833.

I use a scraper to clean off old glue . If I need to thing the bridge on a belt sander I will watch to not thin the wings too far .

_________________
John Hall
blues creek guitars
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