question on veneering, pulling a panel flat if slightly bowed

[mound]

Hi, A question on how to proceed...

See the drawing below to clarify my ramblings..

I made up a 5/8" thick panel of Baltic Birch (24"x36") by glueing together / vacuum pressing a piece of 3/8" and a piece of 1/4" (because that's what I had and I couldn't find the 5/8" I needed.). I used TBII.

I've attached solid oak edging using glue and dominos around the entire perimeter (HW edging is 3/4" thick x 1.5" wide that was milled square and true.)

The edging is 3/4" thick while the panel is only 5/8" on purpose. The edging is flush on one face and proud on the other, which effectively recesses one face of the panel 1/8")

For some reason I'm unclear on, the panel slightly bows out in the center (out away from the recessed face).  I hadn't even noticed until after the edging was done. (Maybe 'cause of having laminated the two sheets of BB?)  If I lay a straightedge down the center line, it's a bit shy of 1/8" higher in the middle than it is at the ends, but I really need it to be flat. The laminated BB panel itself must have a slight bow in it despite the hardwood edging. 

Anyway, I still need to put a 1/16" show veneer over the non-recessed face of this panel (the veneer will cover the HW edging). I'm hoping that if I set the panel in the vacuum bag on a flat table, that because there is that 1/8" void below the whole panel, that the vacuum pressure would press it flat, and then glueing the 1/16" veneer across the whole thing will keep it flat.  Or would it just pop back up?

This is my intuition at least, but I don't want to waste my show veneer finding out that this won't work and I don't want to remake the panel if I don't have to. Any thoughts if I can pull my edge-banded plywood panel flat in this way?

thanks!

[handi]

During the original press, did you use a platen board?

The problem with vacuum bags as opposed to vacuum tables is that the bag will press evenly from all sides where the table presses the parts onto the table. As long as that table is flat, the parts will be.

I suspect the bow was glued in during the original pressing, and I doubt it can be removed easily.

Titebond can be softened with heat. If you could warm the entire glue up all the way through, you might press it flat againinside the bag. But I am not sure if the glue would soften at a temp that would not damage the bag.

[mound]

During the original press, did you use a platen board?

The problem with vacuum bags as opposed to vacuum tables is that the bag will press evenly from all sides where the table presses the parts onto the table. As long as that table is flat, the parts will be.

I suspect the bow was glued in during the original pressing, and I doubt it can be removed easily.

Titebond can be softened with heat. If you could warm the entire glue up all the way through, you might press it flat againinside the bag. But I am not sure if the glue would soften at a temp that would not damage the bag.

I pressed it in a bag on a flat table with veneer breather mesh on both sides.. I see what you mean though by the difference with a vacuum table.. Hadn't thought of that.

[handi]

I pressed it in a bag on a flat table with veneer breather mesh on both sides.. I see what you mean though by the difference with a vacuum table.. Hadn't thought of that.

Using a thick flat panel inside the bag and under the glue up (called a platen) will help keep things flat in the bag. 

We used to make them up with a grid of grooves in the top so mesh was not needed.

[mound]

Using a thick flat panel inside the bag and under the glue up (called a platen) will help keep things flat in the bag. 

We used to make them up with a grid of grooves in the top so mesh was not needed.

yah I've seen what you are describing.. makes total sense now but I had it in my mind for some time now that the mesh did the same job.. I don't do enough veneering to have learned

[jteneyck]

I think your problem came from using two different thicknesses of plywood.  The glue line is off center so the panel warped towards one side.  It might go away on it's own as the moisture from the glue equilibrates and leaves the panel, but probably not.  If it doesn't, I think you're toast unless you can heat it up enough to get the glue to soften so you can repress it flat, as already mentioned.  

Your drawing shows you plan to only veneer one side.  Bad idea.  You need to treat both sides equally to avoid warping.  But who knows, maybe you'll get lucky and the veneer will counter balance the warp that's there now.  I doubt it, however.  If that's your only piece of veneer for the project I would make a new substrate, and I would both sides.  You don't have to use fancy stuff for the bottom, but it needs to be similar species and thickness.  And fill the cavity on the bottom with a piece of plywood, etc. over the veneer so the pressure is equal everywhere under vacuum.    

Always used balanced construction.  

John

[mound]

I think your problem came from using two different thicknesses of plywood.  The glue line is off center so the panel warped towards one side.  It might go away on it's own as the moisture from the glue equilibrates and leaves the panel, but probably not.  If it doesn't, I think you're toast unless you can heat it up enough to get the glue to soften so you can repress it flat, as already mentioned.  

Your drawing shows you plan to only veneer one side.  Bad idea.  You need to treat both sides equally to avoid warping.  But who knows, maybe you'll get lucky and the veneer will counter balance the warp that's there now.  I doubt it, however.  If that's your only piece of veneer for the project I would make a new substrate, and I would both sides.  You don't have to use fancy stuff for the bottom, but it needs to be similar species and thickness.  And fill the cavity on the bottom with a piece of plywood, etc. over the veneer so the pressure is equal everywhere under vacuum.    

Always used balanced construction.  

John

thanks makes sense.

the cavity is supposed to receive one of those self-healing cutting mats for sewing and my hope is that the whole assembly doesn't exceed 3/4" thick.. I'm now thinking I definitely need to rethink/redo some things a bit.. The whole panel is only attached to the cabinet by hinges at the short end (hinges open) so it needs to stay flat without being held in place externally.

[jteneyck]

Try this on for size.  Get a piece of 5/8" Melamine.  Put your hardwood edges around it and then glue your fancy veneer to one side with contact cement.  Done.  I glued some paper backed veneer onto Melamine with contact cement when I built my kitchen 22 years ago. I had a devil of time getting it off a couple of weeks ago during a remodel.

John

[mound]

Try this on for size.  Get a piece of 5/8" Melamine.  Put your hardwood edges around it and then glue your fancy veneer to one side with contact cement.  Done.  

John

interesting idea. I didn't think Melamine came in 5/8", hadn't even considered it. 


Why doesn't that need equal treatment on both sides? You still have a moisture differential on one side from the glue, no? or is it because it's particle board and not plys, or is it because contact cement and not PVA? If the latter, why not do the same with 5/8" plywood?

[jteneyck]

If you want to be extra cautious veneer both sides.  I haven't had any issues only doing one side, but my panels ended up as part of a cabinet so they had some additional stiffening that yours won't.  

Yes, you can buy Melamine in 5/8".  

John