Official Luthiers Forum!

I'm currently re-finishing a guitar I made about 8 years ago for my son. It's been played hard and the finish had worn off of the neck where the capo was frequently being pulled on and off, there were a few cracks in the top and the back seam had opened. I've just finished buffing up the back and it looks OK except that when you hold it at the right angle under light, you can see a line up the back in the finish over where the seam had been. I wasn't totally surprised as this has happened to me before, but I was hoping that with the pore filling and top coat, the seam would be completely invisible. I don't really understand why this happens because, yes the seam had opened, but it was barely visible, and much less in width than, say, many of the pores that I filled and I can't see those in the finish. Can anyone explain to me why this happens and if there's anything I can do to overcome this in future re-finishes?

Thanks,
Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

What kind of glue and what kind of finish? On my first guitar, the finish (KTM9) showed sinking around every glue seam (Titebond Original). IIRC, the LMI website stated as much.

_________________
sweat the small stuff.

I believe the glue was the LMI white glue. The old finish was the KTM9 and the new finish is polyester.
Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Titebond and other aliphatic resin glues like several days before thicknessing to harden, otherwise they continue to shrink over time and you get a line. Franklin talks about this on their web page.

Hi dberkowitz

Do you have a link to that discussion? Thanks - Mike

_________________
guitarsbygiltzow.com

glue just doesn't stick as Dave points out. Curing glues like tite bond will take some time to stop shrinking. But in your post you point out the wood was opened . A seam will often be the first area to show signs of RH stress. In your case it has opened , so what did you do to close and reglue the joint? It is not easy to hide it once it opened up and the more it opened the more sever it can be.
One reason I find HHG and Fish more repairable. It isn't easy to get a curing glue to reglue. Also what sealer and filler did you use? you have too many variables to say for sure it was any one thing.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Under the paragraph "Machining/Post Processing Conditioning":



http://www.franklinadhesivesandpolymers.com/Wood-Adhesives-US/Wood-Adhesives/Product-Family/Titebond-Family/Titebond-Regular.aspx

I didn't do any more gluing. Prior to taking off the old KTM9 finish, the finish was really showing the seam and you could feel it as well. I'm pretty sure it was a result of several years of going through RH changes. When I stripped off all the old finish, the seam in the wood was just visible i.e. not invisible like I usually get when joining two back pieces. After sealing the wood, I pore filled with the UV cure jell filler. At that point no pores or the seam were apparent. I put on the UV cure top coat which is comprised of 5 light spraying sessions. When I sand back level and up to 2000 grit, the seam is still not visible. It is only during the buffing process that the seam appears. I'm starting to think that the finish shrinks a bit during buffing (from the heat?) but I'm not sure. If that's it, I don't know off hand how to buff any less to avoid the problem.

Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Pat, the buff heat could explain it, but IMO, you may not have gotten it level enough before applying the new finish. I would sand across the ridge to level and check it by applying naptha and looking at it under light. It also requires a sandpaper grit that is course enough to CUT the ridge. High grit papers can ride right over it.

without regluing you will never hide the seam filler isn't a glue so the joint will move and that will show the seam .

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Pat, is there any pores visible around the seam?

I used to do some amount of buffing with the uv poly. You could sand it dead flat, only to have pores and seems turn up during buffing. It was theorized that the heat expanded the filler slightly above the surface, which wold hen shrink to just barely show the pores. The only way around that was to move the guitar more so that the buffs never stayed in the same spot for more than an instant.

All a theory though...

Hi Pat
Since you haven't done any reglueing I'm wondering if the center seam itself is unglued? With the center strip (if there is one)
and braces being in sound order the seam may look ok until the finish is buffed. Worth investigating. If so,....what John Hall said.
Good luck with it.
Ken

My guess is that without the center re-enforcement strip up the inside of the back, the open seam would have been more like a crack. Perhaps I should have first tried to work in some glue. I also think that what Ed has suggested has some merit. I was expecting the same result over the cracks that I repaired in the top of the guitar. However when buffing, I was careful to keep the guitar moving to avoid any heat build up. I was able to get the top buffed out with no seams showing over the repaired cracks. After I did that, I re-sanded an area of the seam on the back with 2000 grit paper. With the finish dull from the sanding, the seam line was invisible. I buffed it again being careful not to let it even warm up, but once it was back to glossy, the seam line was there again. Perhaps I was too late for an easy fix.

Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.