Keeping Glue off the Veneering Vacuum Bag

For clean veneering, you need both a platen and cauls. August 26, 2008

I use a vacuum press when veneering various things. Usually it's 3/4" MDF or plywood as my substrate. Can I veneer both sides at once or do I have to bag it twice? Also, does anyone have a trick to preventing glue squeeze-out from getting on the bag? It's sticking in some places and leaving an indentation on the next piece I press. I cover the paper breaks on the edge and it allows the glue to seep through.

Forum Responses
(Veneer Forum)
From contributor T:
I would suggest that you use shrink wrap on the edges.

From contributor C:
You can veneer both sides at once most of the time. When I worked alone, I'd spread the glue on one side of the substrate, lay down the veneer and then the caul on top.

Squeezing the pieces together, even 4x8's, I'd rotate the piece(s) on edge so the caul is down on the table and the weight of the substrate would hold the veneer flat enough until it was in the bag. Always lay down your backer side first, especially if your face has any kind of geometric matches or inlays.

Where is the glue squeezing out from? You may be using too much glue. You should get just a faint bead along the edges and little to none through the face. If you’re veneering porous material, such as quarter-sawn oak or wenge, you may want to consider an adhesive that has a shell-flour filler which practically eliminates bleed through. CP Adhesives makes a product called MPA veneer designed exactly for this purpose.

From the original questioner:

Glue gets on the poly bag because there always seems to be some squeeze-out or the veneer shifts a little or something happens and the glue gets on the bag. I've been using the Unibond 800 that Veneer Pressing Systems sells which seems to work but I'll check out the glue you suggested.

From contributorCD:
Are you using a caul?

From the original questioner:
Yes. I have a 4' X 8' sheet of MDF kerfed in a grid for air movement. I then place the piece substrate-side down against the caul, veneer on top against the poly bag. Do I need to put another kerfed caul on top, the same size as the piece being veneered, like a sandwich? I thought I would get more even pressure not using a caul on top, with the poly in direct contact with the veneer.

From contributor D:
I think you may be confusing caul and platen. The kerfed piece of material is the platen, which is necessary to evacuate air from the bag. In addition, you need a caul in between the veneer and the bag, and if veneering the bottom, between the veneer and the platen.

The caul can be as thin as 1/4" (I suggest no thicker than 1/2") and should overhang the sides of the substrate by no more 1/4" all the way around. In situations where you are using veneers of varying thicknesses on the same plane, use a rubber gasket (roofing material works great) between the veneer and the caul. Also, melamine works well for a caul since glue doesn't stick as well to it. We buy caul material by the unit, but only when it is damaged or being liquidated. We typically pay between $3 to $7 a sheet for this material. Remember to round over the edges of the caul before placing it in the bag.
Using a caul will prevent indentations in the veneer, and will provide a better and flatter lamination.

From the original questioner:
The problem is that I haven’t been using cauls when I should. That explains all the glue sticking to my bag and platen