Two Ukuleles

[Nicco]

Hey brains trust, after a slight shed time deviation, its time to get stuck back into dad's uke. Unfortunately, that means I have to face the problem I'd been putting off... the back panel is really badly warped. 

Interested to hear any suggestions on how to save it, if it can at all. 

A bit of a recap of what we're looking at, it's a 3 ply laminate, 0.6mm qld walnut on the face, 0.5mm qld walnut cross grain in the middle, 0.6mm tasmanian blackwood on the inside. I glued it up on a flat panel, using white pva glue under vacuum. 

First thoughts are obviously that the middle ply shouldn't have been the thinnest, I didn't actually think to measure until after it was all done; silly me. 

Everything was dry when I glued it up, so wondering if the moisture in the pva has contributed too? 

Lastly, it's bloody hot and bloody dry here at the moment, when I glued them up, there was a lot more humidity in the air. Probably a big factor. 

Similarly, the top had warped too, it's actually popped the glue joint on the last 10mm or so of the two ribs. 

Worried that both will be scrap at this point. 

Back panel:

 

Soundboard: 

[Nicco]

I really must clean my bench too... very unphotogenic! Ha ha

[Bizman62]

You might be able to straighten them by applying moisture on the cupped side and clamping the boards between straight slats. Veijo, our tutor, does that every time he calls it a day. Just strips of chipboard with rubber bands at the ends and the boards pinched cross grain between them.

For the laminate even a clothes iron might do as the glue would melt and re-stick when cooling, that's a common way for applying thin veneers. Some steam on the cupped side might help.

Notice how often I used the word "might"... No guarantee. But fact is that the side that gets more airflow dries to a cup.

[Nicco]

Thanks Biz. Yeah, putting some moisture on it was definitely along the lines I was thinking. Steam or an iron are good ideas too. I might start at least intrusive (moisture) then work my way up from there. 

I like the idea with the slats too, thanks. I'll do that as well. 

Worst comes to worst, I have a backup plan for the back of the instrument. Soundboard not so much, but I'll work that out as I go. Ha ha

[Mr Natural]

Nicc0- congrats on the GOTM.

what type of glue did you use on those braces?  I see the squeeze out-it looks like yellow PVC- by chance did you maybe have too much squeeze out? if that is PVC- I would have expected the top wood to fail around that area vs the top wood peeling away from the brace like that- thus my question if perhaps the glue completely squeezed out. I cant tell from the pic but it doesnt appear any of the brace wood broke off and is attached the top (ie- clean break). 

regarding the top- make sure your humidity is up in your shop area before you do anything- I agree with @Bizman62, but if you do that and your shop is say- 30% or less humidity- you will find yourself right back with that cup or maybe worse with a split. (ask me how I know :-))

I have flatten headstock veneers that have cupped by placing them on melamine (any slick, flat surface will do) put a piece of wood on top of the veneer and a small weight (a soup can works) on top of that and let time and gravity do its thing. But that was in my garage in summer so- 80%+ humidity 24x7. 

 

[Bizman62]

3 hours ago, Mr Natural said:

I have flatten headstock veneers that have cupped by placing them on melamine (any slick, flat surface will do) put a piece of wood on top of the veneer and a small weight (a soup can works) on top of that and let time and gravity do its thing.

Exactly. I suppose you did that with the cupped side down?

Anyhow, cupping often happens when the piece is stored flat on a slick, flat surface like a melamine shelf. The bottom side is air tight whilst the top continues evaporating any moisture there is. And there's always moisture somewhere! Even in wood that has been dried down to 5% there's that 5% left. Not to mention that even the moisture of your breath will fall down as ultra thin mist on any flat surface. I'm talking about stuff that you can't wipe off as it's everywhere, in the air, on/inside any surface and even in the towels you use for wiping. The thinner the piece the faster and more dramatic the effect will be.

Thus, if you lay a flat piece of breathing material over a sealed flat surface it can only breathe through the top which will shrink and cup - even a piece of paper does that! The same works the other way around, either by applying some moisture on the cupped side or by preventing the cupped side of breathing. Laying the cupped side down or putting some cling film on it and letting gravity do its thing will work to an extent.

That effect can be used to your benefit as well! If you're building a drop/radiused top, place the top on the radiused body and a damp towel on it. The top will soon swell on the upper side and curve itself to the radius of the underlaying body.

[Bizman62]

Here's something I found in my archives, a good way for storing and transporting small boards. Notice how the slats are aligned so that they won't cause extra warping.

[Nicco]

Thanks both Biz and Mr Natural, some good info in both your posts. I'll find an opportunity to respond in detail soon, so for now just wanted to say thanks. 

I did dampen the cupped side of the back plate and or flattened out pretty easily, but unfortunately it rapidly reverted back to banana shape after it dried out. ☹