How do I remove "layers" of PVA glue from bridge area on 1980 Martin D-35

[Jim Glenn]

I'm new here and hope that I'm not asking a question that has been covered many times before.

I purchased a 1980 D-35 recently that has had a history of the bridge coming off.  The bridge was glued down on it when I got it, but popped off after about 6 months. I can see that there have been a number of different attempts to re-glue the bridge during it's life. I purchased a bottle of De Glue Goo and tried using it to soften and lift the PVA glue, but apparently did not get as much off as I thought I had. I tried re-gluing it last week and after letting it set for 48 hours, the bridge came up as I was bringing the strings up to pitch. It seems as though the pores of the wood are sealed by previous glue attempts. I'm hoping that there is a better product or procedure available that I'm not aware of. This guitar has amazing sound when it is playable, so I want to do everything possible to rescue it.

[Hesh Breakstone]

Yeah there is the location thing too but IIRC it was the early 70's Martins when the locating jig was corrupted and no one noticed until instruments were produced with bridges in the wrong location.

Never a bad idea to check anyway.

[Hesh Breakstone]

Yeah you want great wood-to-wood contact regardless of the glue used.

A couple more things to consider:

1)  When we say HHG here on FRETS we are referring to HHG that you mix up yourself, the real deal, and not that awful Franklin bottled stuff.

2)  If a flat bridge will fit the top with only minimal finger pressure to have complete wood-to-wood contact you can sand the bottom of the bridge on a belt sander and remove all of the old glue.  Be sure to have a belt sander with a very flat platen.  We joke about it in our shop saying "know your platen..."  Once sanded with all the residue removed just prior to the actual gluing scrape the bottom of the bridge with a single edged razor blade which will provide a cleaner, less damaged on the molecular level surface for the glue to bite into.

3)  If the bridge needs to have a bit of a radius on the bottom to fit with minimal finger pressure scrape it to shape with a single edged razor blade. Or if your good enough, I'm not, use the belt sander to create that radius.

4)  Regarding that thick finish the bridge needs to sit directly in a pocket where all of the finish has been removed without sitting up on any ledge of finish.  If all the finish has not been cleared away from the bridge foot print area either do so or clear the finish to within .005ish" of the bridge perimeter and then slightly rabbit the bridge bottom for the thickness of the finish and inset of that .005ish" so that again you have wood-to-wood contact and as much gluing area as possible.  This is how Collings attaches bridges as well, I do this with my own builds also.

5)  You have not mentioned the glue residue on the top under the bridge.  I use a sharp chisel to clear any and all old glue from the top.  It's a bit time consuming, you also have to read the top runout and use the chisel from a direction that tends to lay fibers down and not lift or chop them off.

6)  Original Titebond works for bridges too although I prefer HHG.  Martin originally used a Titebond like glue for the bridge anyway so nothing wrong with Titebond especially if you do not have the ability to have all of your clamps in place and tight in say 15 - 20 seconds when using HHG.  Folks like me and others tend to become HHG snobs but there are some jobs that our chops may not be all that for and the open time is too short with HHG and it will jell before we can have the clamps in place and tight.

7)  I won't stress a bridge reglue joint for 24 hours and I am also keen to have the RH (humidity) in our shop in the 40 - 50% range too with the temp being 70F or more so that that glue can cure properly.

8)  If you mess up the bridge or if it is too deformed this is not a valuable instrument and there is nothing wrong with making a new bridge and it's easy as well.

Anyway pay particular attention to glue residue on the top.  It's harder to see but needs to go or the thing will lift again.

I'm currently not a DeGlue user so I can't speak to the residue.  We scrape, sand, chisel to bare wood.

[Robbie Collins]

Hesh, how do you make the .005 rabbitt?  As you know,  .005" = 1/200". Not especially tight tolerances for metal work but for wood that's pretty close.

[Hesh Breakstone]

Hi Robbie;

Yeah .005" is tight for woodworking but still doable and the number is not really important here with the idea being to expand the gluing surface as much as possible while still leaving a bit of finish under the very edge of the bridge for cosmetic purposes.

Below are a couple pics of the bridge routers, one is from my business partner Dave Collins that he made and it's what we use and the other, IIRC I copied from Luthier Terry Kennedy and was posted on the OLF some years back.

Same idea with either of them.  Ours has the table radiused to 25' though not critical.  In the one that Terry shows you can clearly see the rabbit milled into the bottom of the bridge.  We take it much closer to the edge but Terry's looks like a lot more footprint than often is found under some of the big name maker's bridges.

In the past when reviewing this idea a few people have pushed back that the ledge makes it difficult to get a pallet knife under the bridge.  Mind you the ledge should only be the depth of the finish thickness. These folks back then were folks who also admitted to never having had the opportunity to try either.  There are a couple reasons why this is not an issue in my mind.  First we only usually, not always.... want to remove a bridge because the uppty thing is already lifting somewhere and it needs to be removed, cleaned up and reglued.  Where the lift is is an access point of sorts.  Second with the expanded gluing surface that taking the gluing area to say .005" of the bridge perimeter provides it's possible that the bridge may not lift in time at all.  We have only had a few of ours ever lift even years later and that was attributed to Fish glue failures and high RH in the instrument's environment.

Several month ago when doing a reglue on a Gibson with this method we did the math and determined that since Gibson did not cut back the finish all that much this method expanded the wood-to-wood gluing surface nearly 40%.  That's a sizable improvement in my mind and should reduce the possibility that this bridge will lift again.

Now the pics:

[Robbie Collins]

Ingenious. At first glance it looks like you turned the Stewmac router base w/ edge guide into a pin router, but you're you're using the edge guide as a fence.

Since finish applications vary in thickness does that mean you have to tweak each bridge? I guess it's okay of there is a .001+ gap between the rabbited edge and the finish as long as the interior sits flush.

[Hesh Breakstone]

Yes that's right Robbie sometimes we have to adjust the jig for thicker or thinner finishes and when Dave's not looking I've been known to increase the inset into the perimeter a bit to something that I can actually see......:)  Fortunately the adjustments just take a minute or so to do.  And yes again it started with a Stew-Mac base and I believe that the fence started life as parts from a Stew-Mac binding cutter for Dremel.

But at least we finally found something that a Dremel is not too under powered for...

Something else that I like about this method is that the rabbited bridge and a properly cleaned bridge patch create a bit of a pocket that the bridge will "snap" into.  Not only is this helpful in properly locating the bridge with slippery (for a few seconds) HHG in play the bridge snaps into the pocket and does not shimmy around when applying clamping pressure.

We also help make the pocket with some masking tape on perhaps three sides of the bridge pocket as well.

[Kerry Krishna]

 Jim, I would put the 'deglue' stuff aside and do all that Hesh has suggested. If you are wondering if all the glue residue is gone and have access to a blacklight , use that to check. Considering all the times that this bridge has been off the guitar, I would be doing every single thing possible to get it permanently on this axe. Hesh's list covers all of that. Good luck and let us know what happens.

 By the way, I also have never heard of the deglue stuff before either. Does it smell of vineger? 

[Jim Glenn]

I agree that Hesh's suggestions are spot on. In response to his first item, I still have a quantity of 192g hide flakes and I only mix up enough at a time for each job.  I learned my lesson with Franklin's bottled glue back around '71 and haven't touched it since.

I wasn't aware that a black light would show up glue residue. It sounds like something that I would like to invest in. I want to be absolutely sure that I've got this thing to the point where all glue is gone before I begin. My goal is to do this job right and only once. This box sounded excellent before the bridge popped off, and I sure want to get back to that point.

This MSDS for De Glue Goo says it is a mixture of 5% vinegar and cellulose. Apparently, the vinegar is expected to soften the PVA. The cellulose seems to be there as a thickening agent to turn the stuff into a semi-paste so that the vinegar doesn't run all over everything. Yes, it does have a very pronounced vinegar aroma.

I'm very grateful to everyone who has responded to this post. All of you have provided me with excellent ideas.

[LARRY KLOSE]

Harbor Freight (and my local True Value) has cute little LED black light flashlights for somewhere between $2 & $4.  Cheap insurance and a very useful tool.  I have one but I haven't opened it up yet.  It takes either AAA or AA batteries.

Larry

[Kerry Krishna]

 Jim, blacklights are super cheap.  Bulbs used to be available at Radioshack, but these days I would be surprised if anywhere other than an actual 'lighting' store has them. You can even buy handheld  6 inch battery powered ones for under 20 bucks. 

[Jim Glenn]

Thanks Larry and Kerry. I can't believe that there is a tool at Harbor Freight that I don't already have. I didn't realize that black lights were so cheap. I guess that I haven't kept up on them because I didn't see a use for one until now.

Jim

[Mike Kolb]

and when you're not looking for traces of old glue, the black light flashlights are indispensable for finding where your kitty has decided to mark his territory :)