Joining the back

[Super72bb]

Hi all,

I have a rosewood back with a herringbone strip that I want to join up. I have planed the back halves on a shooting board and then put the best edge on them I could with sandpaper on a flat surface. Holding them up against a window the two halves look pretty good for being true against each other. When I place the herringbone strip in between the two halves I can see the slightest shade of light coming from a few places along the joint. I checked with a caliper and there is a slight difference in width along the length of the strip. I do not have the tooling to true it up. My question is:

If there is a slight gap that lets light through the joint is this still too much? The amount of light is very minimal and I would say you have to move back and forth to even see it just at the right angle. My guess is that this will be easily filled with the glue but this is my first back assembly so I don't want to mess it up.

Any suggestions to help would be greatly appreciated.

Thanks,

Curt

[Oldsoldier]

Before you go further, flip the strip 180 degrees vertically and then laterally to see if there is a better fit between the three components. Actually, I had a different experience. I was having trouble getting a light-proof match between the two halves, but when I put in the strip, it matched better, with no light coming through; and I got a good bond in the glued joints.

[tippie53]

tite bond will fill that plus you should be using a cross grained spruce support on the plate joint so you should be ok.
you can try and true up the filler but that often ends in disaster.

[Super72bb]

Thanks, I appreciate the help. I'll give it a go with titebond.

I'll try and get some pics up when it's together.

Thanks again.

[MaineGeezer]

Please enlighten me....
I had assumed that to put in a back strip one joined the plates, put on the cross-grain reinforcement strip on the inside, then routed a channel for the back strip not quite as deep as the back thickness. That's how I did a line of abalone as a back strip on my first guitar, and a Purflex strip on my 2nd guitar.

From your description, it sounds as though that is not the way to do it, that the back strip is installed as part of the plate glue-up. Or would you do that for certain sorts of back strips but not others?

Will a more experienced builder please expound on this?

[tippie53]

there are many ways to do this.
I used to rout a channel but soon learned that why add a step to a process that doesn't need it. Martin does the back strip between the plates since 1833 . The cross grain adds a good bit of support.
On a 2nd note when I do use pearl in the back I will make a strip and put the pearl into that , then glue that between the plates.

I will make a note and do a plate glue up vid for you guys.

[Diane Kauffmds]

That would be great John. I was also wondering how to do a back strip. I would have joined the plates then routed. I didn't think about the zipper.

[Bob Gleason]

Much easier to do it the way John said. Avoid the extra step. It can be amazingly difficult to route a slot to get a good fit for a back strip. If you just include it in the glue up, then you can sand it flat and do the finish sanding just like normal and add the cross-grained reinforcement after the thickness sanding is done. Also, if you screw up somehow, you can just cut the center strip out and redo without affecting the width of the back. Occasionally your marquetry strip might not be thick enough to match the thickness of the back, in which case you can add a little filler strip under it in the initial glue up.--Bob

[Super72bb]

After a few attempts using different techniques, here is how I finally got the back halves and centre strip true. I used a good quality level with a machined straight edge on it. I placed 3/4" masking tape over the machined edge and cut strips of 220 gr sandpaper. I CA glued the sandpaper strips on to the masking tape. I worked both halves of the back with the level until they were perfectly flat. Once I had both halves true, I placed the strip in between and taped it to one half. With the strip taped to the one half I was able to run the level back and forth to get the strip true. I did both sides of the strip this way and it worked super. In the end I was able to get it so no light was coming through at all.