Dark Glue Line in Scarf Joint

[John Kingma]

I'm shaping a neck for an SG at the moment and as I have been removing wood, my glue line is getting more visible. I know the two faces of the joint are flat and that I have a decent joint but I've never had such a dark line before.

The only thing I've done differently here is that I used epoxy instead of the normal wood glue I usually use. When I was ready to glue the joint I realized my bottle of wood glue had spent a few weeks out in the unheated shop in the dead of winter so I decided not to use it and opted for the epoxy instead. The fretboard is also epoxied on... which is what I always do anyway.

Has anyone else used epoxy for a scarf joint before?

Should I attribute the dark glue line to the epoxy or should I be worried about the joint?

[Bob Gramann]

Many, many years ago, I used epoxy (Smith's all-Wood, I believe) for a scarf joint just like that on an experimental guitar I was building for canoe camping and possible water exposure. The glue didn't leave a visible glue line but over about a year, the string tension put a little creep in the joint to the point where I could feel it when I played. I haven't used epoxy for any lutherie applications since. (I took that guitar apart and did not rebuild it.) That was the only scarf joint I ever made on the shaft of the neck. All of the others, before and after, start under the nut.

[Mark Swanson]

Epoxies i the past were not as good and did have a lot of creep. If the stuff you used was good, don"t worry about it. The wood looks like mahogany, it darkens from the epoxy but will darken just as much when you finish it and that line will go away, I think you are fine.

[John Kingma]

Thanks.

I'm pretty confident that the quality of the epoxy is good enough. It sure cost a lot.

And the neck will be stained fairly dark, so the visibility later no shouldn't be an issue... I was just second guessing the decision to use epoxy in the first place.

[Mark Swanson]

I usually put stains or tints in the finish itself so I have avoided this problem, but if you are staining the wood itself the epoxy line can sometimes seal the wood and not let the stain soak in leaving a light spot. Glue up a piece of scrap to test on, or tint the clear basecoats instead of the wood.

[Pat Foster]

I wonder why the line became more apparent as you worked the wood.

The visibility may just be a characteristic of epoxy. I know an electric builder who uses epoxy to build layers of different woods on his solid-body electrics. With a background in airplane propeller design and manufacture, he knows epoxies like nobody's business. He had the same issue with his guitar bodies, especially where the angle of incidence was low, like on the back of your neck ( in contrast with the sides of the neck where the angle is not as low). The only solution he found was extreme clamping pressure, on the order of thousands of lbs/sq in.

Hope that helps.

Pat

[Barry Daniels]

It certainly suggests that one end of the joint opened up a little before the glue cured. And that is not unusualy with a scarf joint as they are notoriously hard to clamp. They tend to slip a little since the clamping pressure is off axis. But epoxy is a good gap filler so you should not have any problem with this joint.

[John Kingma]

The joint didn't slip or come apart. I use a couple screws to hold the joint together, plus 4 to 6 clamps as well.

Here is a photos from a neck I built a year or so ago.

The screws are in the "waste" area on the edges. I use course thread pocket hole screws that have a lot of clamping power and keep things from slipping.

[Barry Daniels]

Just talking out loud here. I have some clamps like that and mine provide very little clamping pressure. I would prefer screw type clamps on a scarf joint. I also wonder if the screw could cause some slight separation due to the puckering that sometimes occurs in the wood around the screw.

[Randy Roberts]

I'll echo Barry, I can't picture much pressure generated by the squeeze clamps.
Also, the pressure any clamp generates projects out from the edge of the clamp's face at about 45 degrees. Without a caul to stand off the faces of the clamps and let this 45 degrees extend out across your entire gluing surfaces, you may not have much pressure at all at the middle of your neck (where it appears the epoxy line is thickest).

[Mario Proulx]

I hate to pile-on here, but yeah, those clamps are not the best. I won't even use them to hold parts together while I get the real clamps in place anymore(yes, I had some). Just not enough power there. The screws, on the other hand, likely provided excellent clamping, but being where they were could have actually separated the joint, slightly.

Note, could have....

Your joint looks good; any join that is at an angle will be multiplied(visually), and any change of angle will accentuate any gaps, but your glue line appears pretty consistent, and is at a very low angle. In other words, like Mark, methinks you're fine. Plus, epoxy likes gaps... Keep going forward with this neck; I doubt it will ever give you any trouble.

In the future, going forward, get yourself some better clamps, don't use any screws, and yes, I'd suggest placing the scarf under the nut, and not in the shank. This may be the stronger placement, technically, but if there's any creep, you'll not feel it if its at the nut, and that joint has proven perfectly acceptable.

[John Kingma]

I normally DO use screw clamps. The picture above was actually the only scarf joint that I did not use them because all my screw clamps were being used to clamp the cap to the body. Anyway, the neck shown above is on an electric 12 string and it is holding up just fine. I also had 6 clamps on it but took 2 off for the photo so that the screws could be seen... not that it really matters.

I also don't think the wood screws cause any issues. Besides they are in an area that gets cut off anyway. And they really DO keep the whole joint lined up and prevent slipping.

I'll carry on with the neck and not worry about the glue line... and just hope it is not as noticeable when the guitar is finished.

Thanks everyone.

[Tom Sommerville]

In the future I'd recommend a water-based glue over epoxy-- Titebond, LMI or HHG glues
provide a bond exceeding the strength of the wood, and cure out to a much thinner glue line.
Epoxy is almost all solid, so what you have when you make the joint remains after cure; the water in H20 glue evaporates and draws the mating surfaces together.

I also fear that the different expansion rates between the epoxy and wood are going to haunt you with a tactically noticeable seam regardless what finish you use.

Well, on second thought, maybe an epoxy finish...