strange back problem

[John Arnold]

All good points but why doesnt humidifying the guitar fix the problem as it does with the top?

Humidifying the guitar will fix it....temporarily. Unless you want to continue humidifying from now on, it is IMHO not a viable solution.

Does the finish drying/ shrinking, in this case shellac increase the problem?

Not in my experience. Wood will eventually achieve a moisture content that is in equilibrium with the surrounding air, no matter what finish is applied.

The OP has identified a problem with thicker wood. If there are humidity swings, the exterior of a thick board may have a different moisture content than it does in the center. Once you resaw thick wood, it must be allowed to acclimate with the controlled humidity in the building space. This is true, no matter how old the wood is.

[Glen H]

Originally Posted by bob531

All good points but why doesnt humidifying the guitar fix the problem as it does with the top?
Does the finish drying/ shrinking, in this case shellac increase the problem? Though i would think shellac as quite flexible as finishes go.

The top has string tension working where the back doesn’t .

[bob531]

Hi, Well finished those two non identical twin guitars,the back problem has stabilised more or less and both backs are now level, i hunidified for a week and the weather is less dry now.

Very pleased with the results so far (first ive built for 30 years) took approx. two months off and on.

Goncalo Alves backs/sides, figured Cedar of Lebanon tops and quarter sawn Sapele necks

Both guitars are set up and the specs are almost identical . The only difference is the one with the off quarter back has 4 thou nut clearance at the first fret whereas the other has 2 1/2 thou. How low a string clearance at the first fret can you go?? No buzzing anywhere on either at the moment.

The quarter sawn one sound a little better and easier to play.

Both have 4 thou neck relief and both have ended up with 3 .5/64" action at the 12th on the high E and 5/64" on the low E


Bob
[IMG]P1130715 by bob smith, on Flickr[/IMG]

P1130705 by bob smith, on Flickr

P1130709 by bob smith, on Flickr

P1130729 by bob smith, on Flickr

P1130692 by bob smith, on Flickr