Official Luthiers Forum!

Im nearing the home stretch of Dreadnaught Ive been building for almost 2 years.
During wetsanding, after nitro lacquer, , I've discovered the seam joining the plates on the back of the guitar is separated about half way up, between the neck and tail, starting at the purfling (the back has purfling). I would add a picture but

I put a humidifier inside the body and the separation closed up. The only way I even saw it was that the white sanding residue from wet sanding landed in the seam, making a white line. I could feel the seam with fingernail, its was ever so slight. Now after humidity, its even ever more slight, but sill there. I should add that my shop humidity has not been below 35% .

So, I don't want this seam to show, dry or not, so I'm asking if I should seep some thin CA glue down the seam then fine sand (Super Glue Drop fill). Or, should I just drop fill the seam with lacquer. The back is completely lacquered (it was virtually finished). It is the slightest of separations, but separated none the less. Not sure why it took 1 1/2 year to separate, or how it even could separate with the center strips glued between the tone bar braces on the back (directly under the seam). I have no experience to draw from here....

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Measure Twice,

Karl Borum

Wow, that is a large image - sorry...

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Measure Twice,

Karl Borum

What type of glue did you use originally to glue the plates together with?

Plates were pre-joined, from a StewMac kit so I don't know. I will inquire.
Drop fill with lacquer or cyanoacrylate? Or both?

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Measure Twice,

Karl Borum

I'm not 100% convinced that this is a seam separation. Drying out doesn't seem to have been an issue. I think this might be only as deep as the finish. I am no finish expert but I did run into something that looked like this the one and only time I tried to use a water-base finish (poly-whey). I still don't know exactly what happened but at some point during the finishing stage my glue line started to show very faintly. It was almost as if the wood swelled a tiny amount but the glue line and wood right next to it did not. The joint was very tight and well glued. I ended up leveling the finish and french polishing once it was flat and the issue never returned.

I know you aren't using water-base finish but you did use a water-base filler. Maybe our experiences are related. Or, maybe I'm all wet and this is a seam separation.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I'm kind of with Bryan on this one. The only time I've run into actual seam separation is on older instruments where the glue dried up or the original joint was poorly done.

Is there a patch on the inside that spans the join line?

You might ask Stew Mac what kind of glue they used, and what they would recommend. At some point the wood might have dried out, if humidifying it caused the seam to close up , as you mentioned.
If you have a router, another possible fix would be to inlay a back stripe into the centerline of the back, burying the seam under it. Obviously you would have to refinish the back.

Yes, there are cross grain spruce center strips, running the length of the back seam, in-between the back braces. Hope to hear something back from SM soon.

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Measure Twice,

Karl Borum

Any rattle when you tap the back?
In my limited experience with back seam separation the cross grain reinforcement has come up on one side before the seam can open (and often a loose back brace). Additionally perplexing if that is under the neck block. It would be worth probing inside the box with the thinnest feeler gauge you have on hand.

StewMac said they use original tight bond to join the plates. The guy I talked to thought I was seeing wood and glue and finish expansion / contraction at different rates, causing a seam to be visible. He indicated it was likely lacquer moving most, exposing the original "grain line" like happens with lacquer over grain. I've masked a thin strip down the seam and drop filling with lacquer. Will scrape with razor and wet sand smooth gain. Thanks for the replies - Bryan et. al. nailed it.

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Measure Twice,

Karl Borum

Well, still working the seam. Lacquer would not seep into the tiny void, applying with a thin artist brush. Seeped super thin CA into it then followed with GluBoost crystal filler. It built up nice, but after finishing the wet sanding, something shrunk, leaving a very visible trough down the seam. No biggie I guess, now I'm just building it up gain with lacquer via the artist brush. Razor blade and tape scraping, sand to blend and I hope this is a valuable repair lesson I can move beyond.

BTW, used the white sign painters tape that StewMac sells to protect the surrounding finish from drips. Dabbed my brush on it to knock off the biggest part of lacquer drop, IT IS NOT IMPERVIOUS TO LACQUER !! The Lacquer drop seeped right through the tape and left a nice mound on the wet sanded finish it was protecting.. Another Lesson learned, you've been warned.... : )

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Measure Twice,

Karl Borum