Preventing veneer from curling

In the past I've used veneer very sparingly and only on tiny pieces (box lids); no problem. I just tried my first larger piece (cabinet back), and had to use 4 pieces of veneer aligned properly... It all went to^&$^#!

I put PVA glue down on the plywood then placed down the veneers. My expectation was that I would get a few minutes to align them, flatten them, etc, before pressing them down (heavy marble pavers).

However, within seconds the 4 pieces began to curl away from the backing and wouldn't stay down. I ended up scrambling to weigh them down, not being sure whether I managed to get out all the "bubbles" etc. I assume the glue introduced moisture which caused this.

Next morning, I could see that there were gaps, bubbles, it was all crap.

What can I do to prevent (or at least deal with) this next time?

Thanks!!

Wet the veneer first, if only on the non glue side or use a fast tac glue.

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Originally Posted by genericuid

What can I do to prevent (or at least deal with) this next time?

Thanks!!

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Learn the right way first :) . Here's a good start .

Practical Veneering Paperback Charles Harold Hayward | eBay

Use epoxy or polyurethane glue. A vacuum press is ideal but of course not many people have easy access to one.

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Originally Posted by auscab

Learn the right way first :) . Here's a good start .

Practical Veneering Paperback Charles Harold Hayward | eBay

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Thanks (I think! :)). Any quick/simple tips?

I'm reading online that hide glue might be a solution here. I have some titebond liquid hide glue (not hot), not sure if that will help.
Also reading to use PVA, let it mostly dry then iron it on?!
Also reading NOT to wet the veneer.
Also reading NOT to use polyurethane :)

So many conflicting opinions.

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Originally Posted by genericuid

Thanks (I think! :)). Any quick/simple tips?

I'm reading online that hide glue might be a solution here. I have some titebond liquid hide glue (not hot), not sure if that will help.
Also reading to use PVA, let it mostly dry then iron it on?!
Also reading NOT to wet the veneer.
Also reading NOT to use polyurethane :)

So many conflicting opinions.

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Well quick tips could lead to a quick trip to the Tip :) .


Hot Hide glue.
Its an involved process that takes a fair bit of explaining and practice to get working well .

Its not very easy to learn how to lay by hand and join multiple sheets as they go down with an online explanation . You've got to be standing and helping someone do it who knows IMHO .
But here goes. A quick tip .

Basically you moisten veneer, brush the glue down to Ply, put the sheet down run an iron over the top to re heat glue , press glue outwards with a veneer hammer , put some glue on top to help seal and lube, lay next sheet on 15mm over the first sheet do same with glue as first when its good run a sharp blade through both pieces at the 15mm over lay , re heat and wipe away top off cut , little more heat and lift top sheet at edge and remove bottom off cut . Stick back down and run over with veneer hammer.

PVA and heat is only for patch repairs IMHO . I never use it for patching or laying by hand . Ive pressed it and it was great.

Wetting veneer is needed sometimes. Depending on glue and technique your using .

Poly is fantastic for some veneer jobs . But a screw press that's up to the job is needed. Or clamps and cauls with clamps spaced properly. Not Vacuum . Vacuum works but is border line an under achiever IMHO. I have one but prefer screw pressing . Vacuum is possibly not good enough for poly with its expanding thing .
I like using poly on parq top table tops with a simple one piece screw press I made . Its for the tops where the parq pieces are 150 x 150 roughly . Sawn from solid wood at 5mm thick . Or sliced veneer could be used .


Basically though.
If you want to make life easy with flat veneering , make a press.

If you can weld it can be done quick .

Rob

I use quite a different veneering technique than those mentioned above, but mine works for me.

AXIOMATIC: Always do a dry run of the clamping process to ensure that everything is at hand - avoids panic as glue dries too fast!

My technique is as follows:

• Assemble dry veneers on clean bench or MDF sheet,
• Tape over all joints with paper tape - the stuff you lick - available from art supply places,
• Place "cabinet back" on bottom caul,
• Roll PVA glue over "cabinet back"very evenly and sparcely,
• Position veneers on wet glue, paper tape side up,
• Position top cauls,
• Clamp securely - leave overnight,
• Dampen paper tape and it will peal off very easily.

My preferred "inner caul" is UHMWPE - glue is easy to remove - or just MDF wrapped in kling-wrap. I usually layer up 3 or 4 thicknesses of cauls to more evenly spread the clamping pressure.

The more clamps the better!

I also use vacuum clamps, and my experience seems different from Robs. A 90% vacuum means a clamping pressure of 13.2 psi, or about 1,900 lbs per square foot, or over 9,250 kgs per square metre. I cannot achieve these pressures with clamps. For small vacuum jobs, less than 300mm wide, I just use a food vacuum press - works fine.

Have also successfully used epoxy, but PVA is just as good in most cases.

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Originally Posted by GraemeCook

I also use vacuum clamps, and my experience seems different from Robs. A 90% vacuum means a clamping pressure of 13.2 psi, or about 1,900 lbs per square foot, or over 9,250 kgs per square metre. I cannot achieve these pressures with clamps. For small vacuum jobs, less than 300mm wide, I just use a food vacuum press - works fine.

Have also successfully used epoxy, but PVA is just as good in most cases.

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The big numbers are impressive Graeme . There always used to sell Vacuum and its fantastic pressure.\

Cant argue with them and its been done to death years ago .

The problem I have with it is . If I have ten table tops to glue up and I use vacuum . I'm standing there at the end up to my chin in plastic .
And its cost me $.

Out of those ten tops it only takes one invisible leak to give me big problems or lose a job .

Out of ten tops I'm sure Id have that problem more than once.

With a screw press .

No plastic, No waste .

100 % success rate.

Vacuum would be not so stressful on smaller work . Under 1 sq meter. Over that, like what I do sometimes. I never want to try that again .

Broken down the other way, if I got it right, using your numbers vacuum gives 231 grams for every 5mm square . A 50 x 50mm square gives 23.1 kg

I like looking at it that way . And thinking how well I can get a caul to spread that.

Thank you both. From your descriptions, I think the key to my previous failure was trying to deal with 4 separate sheets to be glued on at the same time.

For the next run (where I need 2 sheets per face), I will FIRST join the two (using tape and/or a glue line), then treat that as a single sheet to lay on the plywood. That way even if it curls, at least I'm not wrestling with different sheets going in different directions!

Haven't decided whether PVA or liquid-hide will be preferable...

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Originally Posted by auscab

The big numbers are impressive Graeme . There always used to sell Vacuum and its fantastic pressure.\

Cant argue with them and its been done to death years ago ...

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Yeah; and the numbers are essentially meaningless as you only need moderate pressure for a successful glue up.

But then I do not have the commercial pressures and have not done table size veneering (apart from coffee/hall tables), and so far have not had a serious vaccum leak.

If it works, its right!

For cauls, my answer is simply an extra layer or two of 16mm MDF.

It's been a few years since I did any veneering but pva would not be high on my list except for very small panels. I have used gluefilm in the past and also hide glue but my preferred method is epoxy resin with a vacuum bag. I was gifted a vacuum pump and had a motor trimmer make a large vacuum bag at a cost of $250. Using epoxy gives you what seems an eternitity to fiddle with the veneer positioning and causes minimal curling of the veneer. It takes some practice to get the hang of it and you can even veneer both sides at once if you are feeling brave.