How to remove a bridge? (LMI glue)

Construction and repair of Classical Guitar and related instruments
Patrick delBosque

How to remove a bridge? (LMI glue)

Post by Patrick delBosque » Mon Mar 18, 2013 5:48 pm

I am not happy with the compensation on my experimental 20 inch scale instrument I just finished (the shorter scale requires more compensation) so I want to remove the bridge and "nudge" it a little longer.
What is the best method for heating the glue to get it to loosen up? The glue is LMI Luthier's Glue (white wood glue)
Thanks for any suggestions you can offer.

Alan Carruth
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Re: How to remove a bridge? (LMI glue)

Post by Alan Carruth » Mon Mar 18, 2013 8:43 pm

If you've got a heat blanket you could use that. If not a heat lamp would work, or even a clothes iron

I'd cut a hole just the size of the bridge in a piece of corrugated cardboard to protect the top from the heat, and probably use an aluminum mask over that, too. Don't heat it up too fast: the idea is to get to about 140 degrees, which is the softening point of most such glues, uniformly throughout the glue line. When you think it's hot enough (where's that IR non-contact thermometer?) turn everything off and probe the joint with something like a putty knife. It's best to grind it down as thin as you can for a ways back from the edge so that it's flexible, and will follow the glue line, but don't have a sharp edge. You don't want it to cut into wood, just follow the glue line.

Some folks like to use a hot knife. This works best if you have several knives set up as stated above. Put them in a boiling water bath, and, when they've gotten good and hot, try to work them in under the bridge. When one cools off put it back in the bath, and go to the next one. This works well in a pre-heated joint.

Glues of this type are will also dissolve (more or less) in acetic acid. You can use vinegar, which is about 3% acetic, or photographer's stop bath, which is more like 38%. The acid will react with iron to stain wood black, so use a stainless steel knife for this. If you can get some methyl cellulose to mix with the acid it will form a gel that stays in place better and doesn't evaporate as fast. This is what 'De'Glue Goo' is, so far as I know.

Water, by itself, is a poor choice, hot or cold. Usually the top wood will go to mush before the glue softens much.

If all else fails, or these treatments sound risky (they are, especially if you don't have much experience with them!) you can simply plane the bridge down almost to the surface, and scrape off the rest. Again, protect the top with a mask.

Patrick delBosque

Re: How to remove a bridge? (LMI glue)

Post by Patrick delBosque » Mon Mar 18, 2013 9:35 pm

Thank you Alan. You are a gentleman and a scholar.

Adam S. Vernon

Re: How to remove a bridge? (LMI glue)

Post by Adam S. Vernon » Mon Mar 18, 2013 10:56 pm

With the heating method, wouldn't you have to worry about softening the glue in other parts of the guitar, such as the top braces?

Brad Heinzen

Re: How to remove a bridge? (LMI glue)

Post by Brad Heinzen » Tue Mar 19, 2013 2:49 am

I've found that the LMI glue holds pretty tight when heating for removal. I'd be worried about the braces and the center seam opening up as well. If it were a bridge that I had made, I'd just shave it off, and make a new one. And yeah, I've been there, done that. :) I don't mind taking off fingerboards that I glued with LMI glue, but there's less chance for collateral damage with a fingerboard.

Aaron Green
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Re: How to remove a bridge? (LMI glue)

Post by Aaron Green » Tue Mar 19, 2013 12:57 pm

I would opt to plane the bridge down to almost nothing, make up a shield like Alan mentioned (you want that puppy to fit like a glove) definitely use aluminum foil to reflect the heat and then heat up whats left of the bridge. If you get it to 1 or so mm thick it won't take much and you can peel it right off. I would then use a water based stripper to get the glue residue off the wood, clean it with water really good and let it sit for a bit before gluing on the new bridge.

The problem with heating up the bridge in the case of not wanting to replace it, is that the heat causes it to warp like crazy, no surprise there. You have to bend it back to the original contour and then of course refinish it, probably replace the tieblock bone..,'re getting into almost as much work territory as making a new one and then of course you have a greater risk of damaging the top.
Aaron Green, Luthier

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Re: How to remove a bridge? (LMI glue)

Post by Brian99 » Tue Mar 19, 2013 3:24 pm

If the compensation is not too far out, why not fit a compensated saddle? It would save a lot of work..


Patrick delBosque

Re: How to remove a bridge? (LMI glue)

Post by Patrick delBosque » Tue Mar 19, 2013 4:03 pm

Yeah thanks. I'd planned to plane it down to "nothing" before I applied the heat. That would just simplify and shorten the whole process. Luckily the top has not been finished as the thing is a "prototype" made to test my design (which seems otherwise to be sound, as the thing fits inside a "guitalele" case but has a much louder and guitar-like sound and with the 20 inch scale instead of 17 inch plus a classical width fingerboard is more worthy of being called a "travel guitar".


p.s. The current bridge is already compensated slotted.

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Re: How to remove a bridge? (LMI glue)

Post by senunkan » Tue Mar 19, 2013 4:15 pm

If there is enough material on the saddle block, you can always glue a piece of rosewood to patch up the saddle block and re-route the saddle slot backwards.
I did this on my #1 and it works, just that it looks abit odd.
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Patrick delBosque

Re: How to remove a bridge? (LMI glue)

Post by Patrick delBosque » Tue Mar 19, 2013 4:40 pm

Thanks Senunkan...And did you re-rout while the bridge was still glued onto the soundboard? How did you do that?

Patrick delBosque

Re: How to remove a bridge? (LMI glue)

Post by Patrick delBosque » Wed Mar 20, 2013 5:38 pm

Just to follow up:
I chiseled out a channel, removing the old slot and a little more off the front and back edges of the channel. Then I used the same wood as the fingerboard (Asian rosewood AKA Cultrata)to form a new center contrasting wood slot section. It still needs to be shaped and tapered, etc. but it looks like it's going to work. (And I don't have to remove the whole bridge after all!) Here's a photo.
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