Guide to Vacuum-Bag Veneering
Veneer provides the perfect way to add exotic species and special grain to projects. Gluing veneer to large panels requires the application of clamping pressure evenly over broad areas to ensure a good bond with the substrate. Woodworkers usually rely on platens, clamps, cauls, and a hectic scramble to get everything clamped up before the glue starts to set.
Performance under pressure
Vacuum-bag clamping simplifies veneering large panels by using the power of atmospheric pressure to provide clamping force. Atmospheric pressure is always pushing on everything on Earth at 14.7 pounds per square inch (psi) at sea level (and slightly less at higher elevations). That pressure doesn't crush objects on Earth because air inside the objects pushes back to equalize the force—unless you remove that air, a process called creating a vacuum.
A vacuum-bag clamping system does exactly that. By placing your panel glue-up in a sealed bag and connecting a vacuum pump to remove the air, you can put that 14.7 psi to work for you. Best of all, vacuum clamping applies that pressure consistently inside the bag. That means you get a flat panel withno bubbles, compressed spots, or glue-starved areas.
Bag-and-pump systems exist to suit many needs and budgets. Learn more about them in Selecting a System, below. In addition to the pump and bag, you only need a few other items. See In the Bag—and Beyond, below.
Bag it up to glue it down
Before you spread glue on your substrate, make sure your veneer is flat. Burls and crotch veneers commonly have ripples or puckered areas. Eliminate them by spraying the veneer with softener [Sources]. Sandwich the veneer between paper towels or kraft paper, and then clamp it between a pair of plywood or MDF platens overnight or until dry.
After flattening, apply blue painter's tape to the outside face of the veneer to hold any cracks or tears together during clamping.
Get your vacuum-clamping system and supplies ready to go before you spread any glue. You won't have a lot of time to get everything into the bag before the glue starts to set.
1 Place your panel substrate on one platen, and then use a roller to spread glue over the surface (photo, above). We like to wrap the platens in wax paper to prevent glue from sticking them to the veneer and substrate. Also round over the outside edges of the platens. This helps the vacuum bag wrap tightly around the platens and eliminates sharp corners that could damage the bag.
2 Lay your veneer on the substrate and press it lightly into place, making sure to position it where you want it (photo, above).
3 If you're veneering both faces, flip the substrate over on the platen, spread glue, and apply the veneer as you did on the first face (photo, above).
In the Bag—and Beyond
Tools in photo, above.
A - Pump Removes air to create vacuum pressure, and is rated by cubic feet per minute (cfm) of air it can pull through.
B - Bag Envelops the panel during clamping. A fitting provides connection to a hose attached to the vacuum pump.
C - Closure Pinches the open end of the bag closed for an airtight seal. Some systems use a zipper or tape instead.
D - Glue Secures veneer to the substrate. Specialty glues come in light and dark shades to match different veneers.
E- Platens Distribute force over the panel surface and prevent the bag from deforming the panel or substrate.
F - Breather mesh Eliminates air pockets and ensures consistent clamping pressure inside the bag.
4 Position the second platen to sandwich your veneered panel, and then slip the sandwich into the vacuum bag (photo, above). You need to create your sandwich and get it into the bag before the glue begins to set, so work quickly but carefully.
5 Seal the open end of the vacuum bag using the closure. This bag uses a PVC tube the bag rolls over, plus a C-channel that clips over the top (photo, above).
6 Connect the pump to the bag fitting and turn the pump on. It pulls the air out and maintains vacuum pressure while the glue dries (photo, above). Glue drying time remains the same inside the bag. So, you can remove your panel from the bag in 45 to 60 minutes.
7 Shut off the pump, open the bag, and remove your panel. Use caution when peeling off any painter's tape, because it may have become stuck to the glue.
Now you can cut your perfectly veneered panel to size to add pop to your project. And you'll know that the veneer is well adhered, so you don't have to worry about loose edges or bubbled areas in the future.
With a vacuum-bag system, you'll also have newfound clamping abilities that allow you to do much more than create flat panels. You can veneer curved surfaces, create bent laminations, and even use the pump to add vacuum-activated holding capabilities to benches and shop fixtures.
Sources: Vacuum pumps, bags, mesh, veneer softener, veneer, veneersupplies.com.
Hand-operated pumps, bags, mesh, veneer, 888-857-7790, roarockit.com.
Selecting a system
Like other kinds of clamps, vacuum-bag clamping systems come in multiple styles and sizes. Think about these factors as you tailor a system to suit your needs:
Pump
Electric-motor-driven vacuum pumps must be powerful to create and maintain the necessary vacuum pressure. Expect to pay from $250 to $500 for a quality pump.
Pumps are sized by how many cubic feet per minute (cfm) of air they can pull, which ranges from 1 to 5 cfm. Generally speaking, the larger the vacuum bag you choose, the higher cfm rating the pump should have.
It doesn't mean you can't create vacuum in a large bag using a low-cfm pump. The pump will have to work harder, though, and the process will take longer. So, a small pump paired with a large bag might not create adequate clamping pressure before the glue begins to set.
VeneerSupplies.com [Sources] owner Joe Gorleski Jr. recommends a 1-cfm pump for bags up to 4×4'. If you're considering a bag any larger, step up to a 3.5- or 5-cfm pump. A larger pump works fine with smaller bags.
VeneerSupplies.com offers another pump option: a kit to build your own venturi vacuum pump that uses air power from your compressor instead of an electric motor.
Bag
Vacuum bags range from 2×2' for small jobs up to 6×15' to swallow entire tabletops. Bag sizes represent the dimensions of the panel they'll hold, so they are actually larger than stated. Thick workpieces may require a larger bag.
Bags generally consist of vinyl or polyurethane. Vinyl bags cost less but are less flexible. Vinyl bags are a good choice for occasional users.
Polyurethane bags are much more pliable, so they conform to shapes more easily. And these bags are easier to handle. Most manufacturers offer two thicknesses: 20-mil or 30-mil. The thicker bags are more durable, yet still more flexible than a thinner vinyl bag.
Like pumps, vacuum bags are a long-term investment that you can reuse many times. Manufacturers offer patch kits to repair small holes or tears.
Breather mesh
This plastic mesh provides a pathway for air to escape as the bag squeezes down, ensuring full vacuum pressure and even clamping in the bag. Mesh also reduces the likelihood of glue bleeding to the surface of the veneer. Breather mesh is sold in large sheets that can be cut into smaller sizes.
Breather mesh also makes it possible to use just one platen on the underside of your panel. Just lay the mesh directly on top of the veneer inside the bag. We like the certainty of the belt-and-suspenders approach for veneering flat panels, though, with the veneered panel between two platens, and mesh on top.
A hand-pump option
If you're convinced to try vacuum-bag veneering, but not ready to commit to the cost, try Thin Air Press systems from Roarockit.com. This company started making kits for skateboard builders and has since expanded its vacuum-bag clamping options beyond sidewalk surfers to woodworkers and luthiers.
Roarockit kits use a hand-operated vacuum pump in place of a motor-driven pump. This requires more work but reduces the cost: A replacement hand pump costs less than $20. These pumps work with the company's bags, but won't work with the fittings on bags meant for motor-driven pumps. Vacuum bags are made from vinyl with reusable sealing tape as a closure. Complete kits come with bags from 14×47" to 40×80".