Larry Ciesla Woodworking

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Vacuum Pressing Veneer

For most veneer applications, the problem is how to apply pressure evenly across the entire surface of a substrate.  Applying veneer to a substrate is, in effect, making plywood.  Applying pressure evenly across the entire face of the substrate is a completely different problem than, say, applying pressure to boards being edge glued together.  It is possible to use clamps for smaller pieces, but as the size of the substrate gets larger, clamps quickly become a huge disadvantage.  They simply to not have the reach to get pressure near the center of the substrate. 

Prior to the invention of the vacuum press this problem was solved by building elaborate screw presses.  Many woodworkers around the world still employ such a press in spite of their limitations.  The biggest limitation, in my opinion, is the rather obvious problem that to press larger panels, the screw press must be built on a massive scale.  The size of the timbers and the number of screws required increase as the press is built larger to accomodate larger panels.  Unless you have a huge workshop, storing the press when not in use is also problematic.

I learned how to work with veneer during a week long class at the Marc Adams School of Woodworking taught by Daryl Keil.  Daryl if the founder of VacuPress.com and he invented the vacuum press system that was used at the school and is also sold by his company.  In the years since, I've noticed that many other vacuum pressing resources have become available on the Internet offering competing vacuum pressing systems.  The Joe Woodworker web site is an excellent resource for anyone interested in working with veneer, and they also offer plans and parts if you would like to build your own.

Vacuum bags are available in many sizes in either vinyl or industrial poly.  Industrial poly bags are more expensive than vinyl and are a better choice if you are going to be doing a lot of pressing.  As you can see in this picture, my bag is a roll that I unroll to expose as much of the bag as I need for a given job.  When not in use, I store the bag in a corner of my shop in the cardboard box it was shipped in years ago.  The great thing about vacuum presses is that they easily break down and can be stored out of the way until needed.

A key component of the system is a "platen" you custom make to fit both the bag and the job your pressing.  Melamine is a great material to make platens from because glue will not stick to the melamine coating.  You can see a cross hatch pattern of kerfs in the melamine that allows the air to be drawn out of the bag thru the kerfs into the vacuum pump (which is not shown in this picture.)  A hose from the vacuum pump connects thru the platen via a nipple on the bottom of the bag.

The vacuum pump connects to the vacuum bag via a stout hose.  This model is very similar to the one I own and has a continuous and automatic mode of operation.  As the pump draws a vacuum, two things happen inside the bag:
The result is very even and strong pressure applied to the substrate and veneer with the glue tending to penetrate deeply into the cells of the veneer and substrate. 

I have found that the best glue (by far) for pressing veneer is Urea Formaldehyde.  The specific product I use is called Unibond 800 and is available on the Internet.  This glue is excellent not only for veneer and marquetry work, but also for gluing bent laminations.  It has a long opentime, forms a rigid glue line, and cures quicker with heat.  It is a two-part glue consisting of a liquid resin and a powder hardner that you mix just before use.  Once mixed, depending on the ratio used and the ambient temp, you have at least a 1/2 hour open time, often longer.  To speed up the curing process, after the vacuum is applied I cover the work with a heating blanket.

The worse glue to use for veneer is contact adhesive!  I'm amazed how many times I've seen "experts" on woodworking TV shows teaching that contact adhesive is appropriate for veneer work.  It is not, and is almost always a very poor choice.  Contact adhesive was formulated to work with plastic laminates, not veneer.  It is prone to form bubbles on the surface, will soften in the presense of heat and alcohol, and some finishing solvents can penetrate the thin veneer and weaken the adhesive bond.  Furthermore, it is impossible to reposition the work after making contact with the glue, and worse of all, it only forms a surface bond and does not penetrate the wood fibers. 


The taped side of the veneer is the "show" side.

The side without veneer tape is the glue side.  I only point out this obvious difference because, even though I do this a lot, I have screwed this up!

I make two cauls, one for each side because I'm gluing both sides at the same time.  Because these are very large cauls and a large panel being glued up, I covered each caul with plastic film.  Taping the film to the MDF before the glueup procedure makes life a lot easier!

Since this is is going to be a rather large panel, I use a full size foam roller that will make it easy to place an even coat of glue on the panel.

Urea Formaldehyde glue is not something you want to get on your hands, so gloves are an absolute must.  You'll see the what I do with the blue tape in a bit.

I make a mark on two cup to show roughly a four to one ratio of resin to hardner.

I pour a small amount of hardner into the resin and begin mixing.  The idea is to mix the hardner in a little at a time so you don't end up with a lumpy mess.

Mixed and ready to go.



I pour some of the glue and spread it around with the roller.  The result should be a thin and very even coat as shown in the bottom photo.

Glue has been applied to both faces of the substrate, the bottom and top veneer are in place, and the "sandwich" is placed on one of the pre-prepared cauls.  I use packing tape to secure both veneers to prevent movement as the press starts to apply pressure.

I secure the top and bottom cauls together using blue tape.  I try to make the package as secure as possible so nothing moves during the pressing process.

The package is heavy so I move it around the shop on a wheeled cart that height is perfect to allow me to slide it into the press.

The bag has been sealed, the pump turned on, and a vacuum drawn.  The Urea Formaldehyde glue is a slow setting glue so I'll leave it in the press all night.  Before I leave I'll cover the panel with a heating blanket.