Headstock cupping problem

[vandenboom]

I prepared these necks many months ago but just discovered this cupping of the headstock. It is most pronounced on the edges.
The left shaft is Fijian Mahogany with a Wenge strip up the middle - the cupping is more severe in this case.
The right shaft is Blackwood with Silver Wattle strip up middle.

I imagine the thin veneers are responsible for curling the edges up. Does the laminated strip up the centre of the shaft create more weakness across this direction than a single piece of wood, or do you think there is something else going on.

I'm not sure what to do about it - whether to remove the veneers, reflatten it and apply something different, or apply it in a different way. I don't like removing wood in these situations as I often seem to cause some form of other irreversible damage!!
Thoughts appreciated....
Thanks. Frank.

[Kim]

Hey Frank,

My first port of call would be to look at how these have been stored. If they were glued up and left sitting with the overlay veneer face up exposed to hot dry air from a tin roof or something that could be 'part' of the problem. For now I would wet the veneer a few times and watch what happens over the next day or so, wet and watch, wet and watch. If it pulls itself flat leave it to stabilise and then seal with shellac to work as a buffer.

Having said that, the first rule of veneering and to a point 'finishing' comes to mind. That is, what ever you do to one side of a board, you need to do to the other if you want it to stay flat. What you show makes a great argument for applying a back veneer to the headstock.

Cheers

Kim

[vandenboom]

Thanks Kim. Unfortunately, I haven't read anything much on veneering, so I was unaware of that one, but it makes sense now.
As far as a hot room goes, my 'shed' is fully insulated but not air conditioned, so it would have been pretty hot and dry in there on some of our hot Melbourne summer days.
I will do as you suggest - wet and watch.
I'll also have a look to see if applying a back veneer at this stage can still be done - not sure.
Thanks. Frank.

[kiwigeo]

Might just be my eyse but isnt the whole headstock slightly cupped?? This would suggest veneer isnt the problem. Just my ten cents worth.

[vandenboom]

Might just be my eyse but isnt the whole headstock slightly cupped?? This would suggest veneer isnt the problem. Just my ten cents worth.

Yes - that is the case.
I can see that the veneers drying out are at least part of the cause, but wondered if the lamination up the centre of the shaft loses some strength across the headstock.
After gluing up the laminated neck and before applying the veneers, the headstock was flat.
Timber in both necks was also kiln dried.

Frank

[Localele]

For my ten cents it is applying veneer to only one side.Having said that and looking at the pictures I would just sand it flat and get on with the rest of the building.Hard to tell with pictures but I don't think anyone would notice the slight taper in the headstock overlay caused by sanding it flat.

[obmit]

Kim is dead right.

Not only veneer does this but also lacquer. I have seen this so many times in other applications its not funny.

If you were to lacquer the head stock on both sides you may have gotten away with it as it would have been sealed.

This is a major issue in Furniture making.

you need to stress the timber the other direction and then once flat seal it up.

Good luck I hope it all works out

Cheers

Tim

[Kim]

Might just be my eyse but isnt the whole headstock slightly cupped?? This would suggest veneer isnt the problem. Just my ten cents worth.

Martin,

The layers of veneer and glue create an imbalance in the way moisture in the atmosphere is transferred to and from the solid woods. This alone is easily enough to cause what you see in the images. I have seen a 50mm thick X 300mm wide board of jarrah do the same only the 'cup' was much more exaggerated over the greater width and this was all because of a single coat of urethane varnish on one side only. This same board had sat flat as a biscuit for years prior to being coated. Such an experience is a clear enough lesson to never under estimate the power of wood. It's need and ability to move as RH demands is a completely unstoppable force. If every you did manage to contain this movement, the board would simply self destruct as is apparent when a top is glued on in high RH and the instrument is moved to low RH.

Cheers

Kim

[pat foster]

Frank,
I'm trying to imagine how veneering in and of itself on one surface across this narrow a span would cause this problem. There are thousands of guitars out there with veneered headstocks that aren't cupped. A wide span, like a table top, I could see it. I've had necks with headstock veneers glued on sitting around for months, going through swings in humidity as seasons changed, without cupping, not that I'm doing anything special.

I'm wondering if any of the veneers were glued on cross-grain, which would provide one stable, plywood-like side, and one not as stable, solid-wood side. Any sign of it on the fretboard surface? You mentioned that it had been kiln dried. Did it get a chance to age before you used it? That could be an issue with some woods. I keep my neck wood at least a year before using it.

I would also guess that it's a problem with moisture content of the woods when they were glued up. Or some of the woods involved might be notoriously unstable.

If it were me, I'd try to strike some balance between sanding out some of the curve, but paying attention to how the veneer edges look, perhaps not going completely flat. Might look OK sanded flat too.

Pat

[Localele]

Also another good reason for sealing up the insides of a guitar with Shellac before closing the box. Not only do they look much nicer inside you can help to stop uneven take-up and release of moisture.