veneer curling

[TimberNut]

the project - a jewlery box.
the issue - I need the top and bottom to be quite thin substrate with veneers glued to both sides, and only a total thickness of about 4mm
Here's what I did-
I cut masonite into required sizes (260mm x 160mm approx) and put through thicknesser to get down to about 2mm thick.
one side was to have birdseye houn, and the other, normal houn pine. The birdseye was quite warped so I soaked it in water for about 1/2 an hour to soften. The normal huon was flat so i didn't wet it.
Then i applied titebond? pva to both sides of masonite, and put veneers (cut oversize) above and below. I wrapped in greaseproof paper, and clamped between 2 x 18mm thick blocks of MDF with LOTS of clamps and left overnight.
All seemed fine when i took the clamps off this morning. This one is OK and what I was after
67637
but when i came home tonight, 2 were bowed quite bad.
67639
67638
I'm guessing it's due to excess moisture in the birdseye huon compared to the other side which wasn't wet when gluing????

1. should i have wet all veneers before gluing (so they would have roughly same moisture content) as they dried?
2. can i wet these and clamp to flatten in order to salvage? or will they go back to warped shape?

advise please!
Timbernut

[watson]

G'day Timbernut,
Short answer .....I don't know for sure..........but I dampen dry/warped veneers with a spray of glycerine and water.....both sides of the substrate.
If I had to go to the thickness you required I may have used 3mm MDF to save any movement in thicknessed masonite....even if it (masonite) would warp when moisture is applied to one side.
To alleviate the problem I might have increased the groove for the lid a bit.......to do this I shim the table saw fence with playing cards.
My measurements are:
3mm ply....veneer both sides......table saw groove (3mm) .....shim with 4 playing cards.
Hope that makes some sense.

[TimberNut]

Hey Watson,

Thanks for that. Unfortunately due to box design, I can't increase the overall thickness of the veneered boards (first box ever, and I'm still learning). This will be one of the (many) things I will look to change in subsequent box designs.

One of the 3 boards I did only had a veneer glued to one side, as the other will have a veneered pattern assembled for it. I thought it'd be easier to glue the backing veneer first, as it was just a single sheet, so I was expecting perhaps a little distortion of that one till I could glue veneers on the top face, but not as much as i got. And the other board with veneers both sides - I'm baffled.

Why would 3mm MDF reduced to 2mm be any better than masonite? I would think both would absorb moisture to the same extent, so either would be fine?? I understand, if I could leave the MDF at 3mm, but as mentioned, that is too thick for this box.

Timbernut

[watson]

G'day Timbernut,
Yeah, I probably wasn't real clear in my post last night.
I meant use 3mm mdf as the substrate and increase the size of the groove in the box sides.

[TimberNut]

hi Watson, yeah I got that bit, but unfortunately that will result in the boards being thicker than my box design will allow, as such I'm forced to use a 2mm thick substrate. So it sounds like, considering that , the issue of MDF vs masonite would be a moot point, as their ability to absorb moisture would be roughly equivalent?

Which leaves me with the assumption that my main problem was that I didn't wet the veneers for both sides, but only one, and so as that additional moisture was lost from the wetted veneer, there was greater shrinkage from that side, and so the boards curled in that direction.

I'm hoping some of you more learned veneering gurus can provide pointers to prevent this happening when I try again.

[tea lady]

In my small experience with veneers I think you do have to take notice of the moisture content of all and perhaps even need the substraight to be a bit wet too. Why not use very thin ply, or make your own ply to get exactly thickness you want, making sure you cross the grain evenly to keep it flat. Then you know its real wood all the way through.

If you spray burl veneer with 10% glycerin/water mix and press until dry will miraculously relax bumpy bits and easier to work with.

The books about laminating say to have odd number of layers so that outside grain is going in the same direction. But I have found that odd numbers will warp because three shrink more in one direction then the two in the other.(Does that make sense?) So I have even number of layers.

Hope my humble suggestions help.:B

[powderpost]

G,day Timbernut....
I regularly use burl veneer, which is anything but flat, on 3mm mdf board for box lids about the same dimensions you mentioned. I stopped many years ago from wetting the veneer to flatten it, for the very same reasons you mentioned.. Don't know where the idea of flattening veneer before using it came from, I find there is enough moisture in aquadhere to do the job. Guess for larger jobs e.g. cupboard doors etc. damping with a moist cloth or a very light spray could help, but for small jobs I reckon it is not necessary. It is important the panel be 'balanced' with the similar thickness veneer glued to both sides. Hope this helps you........
Jim

[TimberNut]

Thanks Jim,

I remade the one that I'd put veneer on both sides at the same time as it was pretty much rubbish. This time i soaked both veneers however.

I think my problem was that i didn't just wet the veneer, I soaked it for 1/2 an hour.

Next time I think i'll try, as you say, relying on the moisture in the glue to flatten the burl veneer, and see how that goes.

[tea lady]

The glycerine thing really works. Trust me. So if you still have trouble with curling, spray with 10% mix with water on both sides. Flatten under boards and phonebooks or something untill dry. Voila. Flat. But not wet so no shrinking.

[TimberNut]

Tea lady:
so if i understand you correctly I should
- buy glycerine (I think you get it from a chemist, is that right? is it expensive?)
- mix 10% glycerine, and 90% water, and put in a cheap $1 spray bottle.
- spray surface of veneer (till wet? or just light mist and wait for it to soak in? how much? )
- place between 2 x flat surfaces and weigh down
- leave till dry (overnight or more)

next day veneer should be 1. dry, and 2. flat.
THEN apply glue to substrate and fix veneer in place and clamp till glue dries.

Is that right??

[joe greiner]

...Which leaves me with the assumption that my main problem was that I didn't wet the veneers for both sides, but only one, and so as that additional moisture was lost from the wetted veneer, there was greater shrinkage from that side, and so the boards curled in that direction.

Correct. I can't speak to veneering specifically. But waaaaay back when, for attaching a drawing to illustration board, both the drawing paper and an identical material and size for the back were wetted and glued, more or less simultaneously to prevent bowing. For veneering, similar grain direction for the front and back should likely also be employed because of different shrinkage characteristics along vs across grain.

Joe

[tea lady]

Yes Timbernut,that's right. Can get glycerin in supermarket in medicine section. About $9ish dollars for small bottle. I think there are other threads on here about it too. Using to soak wood for turning and stuff. Might be worth looking up. There is a bit of debate about it but it worked for me.:cool:

[homey]

TimberNut,

I use veneers constantly on my boxes. The veneer should not be wet when you glue up.

When veneer is wet or even reasonably damp it expands. Glue that on, and as the glue and veneer dry, the veneer shrinks back to size and warps the substrate. The thinner the substrate the worse the end result.

Burr veneer (burl to our American friends) is often bumpy. It may or may not need flattening. For example, if the burr veneer is very bucked and also very dry, simply laying on a glued substrate and pressing it between boards often cause the veneer to crack. Other times such as a gently waving veneer no flattening may be necessary.

To flatten you’ll need the glycerine type softener. I buy it ready made up but then I’m lazy. Essentially you spray the veneer, let it soak in for a few minutes then put layers of paper towel either side and clamp between boards. After a couple of hours take it apart, replace the paper towel with fresh stuff and go again. You may need to do this several time over a day to make sure the veneer is not only flat but also dry. Lots of descriptions of all this on the web.



Good luck!

Brian

[NCArcher]

12 year old thread...I think the veneer has curled into another dimension by now :D

[homey]

Sheesh - thanks M’Lord.

Wonder how I came across that post? Only been looking at Activity posts today. Must have let my fingers do the walking!! :-