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  1. #1
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    Default Major cupping on thin hardwood glued to Marine ply

    Hey guys! I'm looking for some help with a recent project of mine (and first of this type). I made a photo album cover using Marine ply (3mm thick) and dry sheoak and spotted gum that I sliced into 3mm thick strips each approx 30mm wide. I've glued these strips Chevron style and alternating between the spotted gum and sheoak and glued together using Sika flex pva glue to create a strong bond. I also used foam rollers to spread the glue evenly across the ply and used weights and a panel to press everything together. However both front and back panels cupped! If someone more knowledgeable than me can explain why this happened and how to avoid it I would much appreciate it! Cheers, Jaff.

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  3. #2
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    Yep, that sounds about right. The timber expands with the changes in the environment, but the ply doesn't so it has to bend. The only way around it is to glue the same thing on both sides of the ply.

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    Thanks for that! It's interesting though. The wood was dry before being cut and I also left it lying around as thin slices for about 2 weeks and they remained nice and flat. I suppose by putting glue on the bottom and sealing the underside forced mositure to wick out the end grain and top surfaces? Hence creating the cupping? If that was the case, would cupping be prevented if I sealed the top too with varnish? Say 24hrs after gluing everything? (they were still flat then).

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    Asking because gluing both sides would make it too thick at 9mm.

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    As elanjacobs said, what you do on one side you should do on the other. Especially when your substrate is very thin like 3mm. Long ago I veneered one side of a piece of 4mm ply with some 0.6mm timber veneer. Within a few days it was banana-shaped. And that was just veneering 0.6mm

    When a piece of timber absorbs moisture it expands across the grain. Likewise it shrinks when it loses moisture. A quick search online will point you to a number of timber expansion calculators where you can see how much a particular type of timber moves. If the piece is just sitting on your bench it will quietly go about its business expanding and contracting. As soon as you glue it to a thin ply it will continue to expand, but because you have prevented one side from moving, the expansion will continue where it can, i.e. on the ‘open’ side, forcing the whole thing to curve.

    Whether you could stop this effect by encasing the whole thing in polyurethane is something I don’t know, Jaff. I hope someone on here can answer the question - I’m interested to know!

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    I don't know of any seal that will work enough to prevent the cupping. I would glue the wood on both sides as suggested by Elanjacobs, then reduce thecovers in thickness by putting them through a drum sander, reducing both sides until the thickness is satisfactory. I'm doing this with a marquetry job I'm on at the moment.
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    Varnishing it quickly might help, but varnishes are still somewhat porous so it won't completely solve the problem. The best (only?) way to stop it happening is to keep the movement balanced by having the same thing on both sides (that includes the same polish on both sides too). If you only do one side, it WILL move; physics always wins.

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    A really simple demonstration of how the moisture absorbs into timber from the application of PVA glue can be gained by applying PVA to 0.6 mm veneer. Most veneers will start to cup immediately and cup away from the glue side - the moisture creates expansion on one side.

    Applying a "veneer" to both sides of a panel is only a partial remedy.

    Controlling or more correctly equalizing the rate of loss of moisture from both sides of the panel as the glue dries is also critical. I have found in my work with inlay banding laminations that after the initial set of the glue and after removal from the cauls & clamping pressure it is critical to place the panel between two similar cauls of say MDF to "buffer" or slow the rate of moisture loss. Leaving it under mild pressure, i.e. some other boards / weights for several days significantly reduces the potential cupping issue. Still not an iron clad guarantee of success though, because in the real world the outer surface of the cover will react to changes in moisture more rapidly than the inner surface which is buffered by the inner contents.
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    To everyone thanks for the replies! There's heaps to think about and I'm keen to experiment a bit more. I'm hopeful my second attempt is more successful since the photoalbum cover (with family photos) was my mum's 60th birthday present and is now spectacularly warped! I don't have access to a drum sander unfortunately, but I'll make a jig to take those extra few mm of with a handplane. I'll think I'll try thinner ply too and also experiment with both sides glued up (with and without poly) and one side glued with poly. And also apply the clamping methods!
    Is it possible to send photos on this thread via mobile? I didn't see any options when I writing up the original post. I'll keep it updated with photos so beginners in veneering (like me) can see what not to do lol.

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    I suspect that a significant factor for the moisture imbalance would be the actual moisture contained it the glue when applied. Some of this may evaporate along the way, but the bulk of it would be absorbed by the timber, and potentially be trapped long term near the glue layer as the glue sets. As previously noted, the ply would be much more dimensionally stable than the timber because the layers are bonded at right angles to each other to resist movement.

    One possibility I could think of would be to glue the timber to a sheet of paper, and then glue up a similar panel in the opposite direction on the other side so the timber strips cross at close to 90 degrees (as close as you can get with the particular angle you have on the Chevrons anyway) to give the visual effect, with similar resistance to cupping that plywood has. Thickness would be similar, two 3mm timber layers vs 3mm timber and 3mm ply. One potential shortcoming that this might create may be that the edge joints might open up a minute amount over time because of differential moisture content between the exposed timber faces and the timber in the glue area. You would need to make sure that that the apex of the chevrons was offset somehow on the reverse side, so that the apexes of both sides don't form a single straight line down the face, which would generate a monumental week point.
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    Just change the glue to something that does not contain water.

    I had a similar problem glueing parquetry to a table top many years ago . I was using PVA or Titebond . The sawn veneers I was laying to one side were cupping before I could get them to stick . I didn't have a press big enough either. So I thought I was smart and wet the veneeers on the back side with water to counter the cupping . So glue one side and water on the side to be polished. They stayed straight . Wow I thought ! solved that . Glued them down . Then the water dried off . The glue was stuck and the whole table top formed into the biggest banana Id ever seen .

    So I did some experiments with thin 2mm cut timber 20mm wide x 60mm long x 2mm thick

    I laid them out and put mt pva or titebond on one side and watched them cup and curl up straight away . Then I got some techniglue two pack stuff which I had thinned, some with metho and some with turps . Tested them on the thin cuts of timber and no change in shape . No water = no change .

    I then went through a stage of glueing parquetry down with thinned techniglue for each job that required it . Its thinned because the glue was to thick left at normal out of the can thickness for what I wanted.

    You could either use techniglue normal or techniglue thinned or poly glue but your going to have to clamp it somehow.

  13. #12
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    Thinner veneers and techniglue or similar. Veneer both sides. Will be solid and stable.

  14. #13
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    out of curiosity. why veneer to ply board? At such thickness why not make it out of solid 6mm thick wood? or laminate 2 x mm solid timber pieces to make 6mm thus avoiding ply and with that capping...

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