Official Luthiers Forum!

I am regluing a bridge on a cheap guitar for a friend. The bridge took some wood with it when it pulled loose - the top layer of wood on an area that I'd say is about 15% to 20% of the total gluing area, located off center but not all the way out to the side. All tear out was on one side of the center line. The tearout is thin - no major chunks came with the bridge. Just the surface peeled off.

I have the bridge and top gluing surfaces cleaned up. There is obviously going to be a gap in the area of tear-out. This is way beyond my experience - how do you fill the gap, and what glue do you then use to glue the bridge back on?

Thanks.

Jim

_________________
Jim Kirby
kirby@udel.edu

This is one fix where actually I like "Liquid Hide glue". I mix up a thick paste of liquid hide glue and spruce dust (a lot of dust) and over fill the tearouts. Then when cured (24 hours) sand down to flush with 100 p. I then use HOT HIDE glue to reglue the bridge in place.

That said 20% -30% is a slot of tear out.

Michael -

Thanks - hopefully 15%-20% surface damage was a real upper bound - it probably looked like the Grand Canyon when I first saw it but gets less bad as you get used to it.

_________________
Jim Kirby
kirby@udel.edu

So the top's laminated? There's usually serious tear-out when the bridge comes loose on these tops. My understanding is that the standard for repair is to trim the torn-up area back to undamaged wood, then inlay a spruce patch into the cavity. Level the inlay, glue on the bridge with your choice of glue.

Using wood dust/filler might be a reasonable solution if the guitar's not too valuable but I doubt that adhesion to the putty would be very good.

Ric - It's a solid top, at least as far as I've been able to tell. I think it's an ebay Indonesian special (like those really cheap Taks), in the second price tier (not a $79 one, more like a $200 one). I'm not quite sure what species it is.

_________________
Jim Kirby
kirby@udel.edu

The adhesion issue is usually caused by un-like glues that is why I like the liquid Hide for the paste filler here and hot hide for the glue up the like proteins adhere well to each other. Yes the best fix is a properly fitted patch graft.

My experience is that regardless of laminated or solid tops the biggest cause of tear out is not opening the joint from the down grain direction. If a top is made up of book matched halves then one side the grain run out direction is opposite of the other. When separating a bridge from the top it is best to always work the hot knife in from the down grain direction. If you work your knife in from the up grain direction you are working into the run out and much more likely to pull fiber out. this means you work the bridge off from the rear one half and from the sound hole side on the other half always working down grain.

Thanks Michael - that makes sense. That would probably explain why all the tearout was on one side when the bridge launched towards the neck. Hadn't occurred to me.

_________________
Jim Kirby
kirby@udel.edu

If most of it is on one side and you worked from only one side I can almost guarantee that is why it tore out. I picked that tip up from Frank Ford many years ago and like you once explained to me it made perfect sense and solved 98% of my bridge repair fiber tear-out fro that time forward.

You can tell the direction of the run out by looking at the two halves with a light source from behind you. The half that is lighter in hue than the other is down grain. This is because the into the run out or5 up grain half will reflect less light back at you

Oh, I'm thinking now that you are thinking that the bridge was being removed when the tearout occurred? Nah, the bridge and guitar parted company spontaneously - I'm just called in to mop up.

_________________
Jim Kirby
kirby@udel.edu

Yea! don't mater the tear out one the one side more than the other was still cause by the direction of the joint failure sheer in relationship to the direction of the grain runout. same end result

If you get the same responses as I got with this question, you will will have NO AGREEMENT as to how to handle this problem. Read this post where I asked the same basic question: viewtopic.php?f=10101&t=23130

Nevertheless, I learned a lot from the conflicting advice.

Ed

I guess you can also use fish glue and spruce dust, and then fish for the bridge attachment. I wouldn't sweat it much knowing the surfaces are prepd nicely.

For a rather scary story, when I started meddling with building a couple y ago, a friend brought his campfire classical guitar with a lifting bridge. It was very clunky made and of maple and I thought about making him a decent rw bridge. There were a lot of big tearout gaps in the top but I didn't fill them at all, just flood liberally with glue. I used LMI white and it still holds well. And he uses medium gauge steel strings BTW and the guitar gets carried up in the hills every month or so including winters. About 1y ago I glued up a side split (about 1/2 of the length) That guitar has a rough life

_________________
Build log

Ed -

Thanks. Now that you've pointed it out, I remembered that thread. I did have epoxy in the back of my mind, but I hate cleaning up epoxy. I'm leaning towards Michael's suggestion.

_________________
Jim Kirby
kirby@udel.edu

Jim,

Since the guitar in question is cheap, and the top is already damaged, what are the odds that the guitar will be worth fixing if the bridge comes off again in the future? If it is unlikely to be worth fixing again well down the road, then maybe epoxy is worth considering. It will be a fast easy fix, gap filling for a "sure thing" approach, and the squeeze-out can be cleaned up with some alcohol while first clamped. The odds are that it will not come up again when done with epoxy. Other approaches (getting spruce dust and mixing) may be more work and may be less reliable.

If the guitar was valuable I would try to avoid epoxy. After carefully considering all the input from the Forum and the condition of MY guitar top, I ended up using Tightbond since it was a 53 years old valuable vintage Gibson ($4k). It worked well for me since my tear-out was not very bad. And the old Gibson could have the bridge fixed again someday after I am dead and gone. I don't think you have the same situation.

Ed

Partial strike out on Michael's suggested plan. I can't find the liquid hide glue locally.

I'm thinking of doing the fill with epoxy and sawdust, and then doing the bridge glue-up with hot hide glue. Does hide glue stick well to epoxy?

The damaged area I need to fill is not really as bad as I had convinced myself or originally suggested here. I think it is about 5% of the total gluing area.

Thanks,

Jim

_________________
Jim Kirby
kirby@udel.edu