No matter how many times you've used a vacuum press, there's always a moment of slight anxiety when you pull a veneered panel out of the press. You're wondering if there will be bubbles, ripples, delamination, or seam separation. Here are a few tips to ease the worry and avoid the most common mistakes when working with veneer.
1: Prepare the Surfaces Properly
For all substrates
For paper-backed veneer
For raw wood veneer
2: Choose the Right Glue
4: A Backer or Balance Veneer Minimizes Warping
One of the most common veneering problems is the tendency of the panel to warp after it is removed from the press. You can eliminate this issue by veneering both sides of the panel. A backer or balance veneer should be used on the reverse side of the substrate. This will even out the stress placed on the substrate as the glue dries and the veneer settles into its final position on the panel. Learn more about backer veneer use on this page.
Small & Medium Panels
5: Vacuum Press the Panel for Best Results
A vacuum press is the ideal tool for clamping veneer to a substrate because it applies evenly distributed pressure across the entire surface. If you do not have a vacuum press, you can usually get a good finished panel with clamps and weights. The key is to make sure the pressure is evenly distributed. Otherwise, the glue may create rigids and bubbles in the veneer. This will ruin the panel. Click here for more information about veneering without a vacuum.
6: Allow the Panel to Dry Correctly
Standard cold press veneer glues require 60 minutes of press time to "set" the veneer glue. Remember that the actual curing takes another 4 to 6 hours outside of the bag. Be sure to let the panel dry completely before you do any sanding or machining.
PPR's and urea resins require 4 to 6 hours in the press. These glues do not require air to evaporate the liquid in the adhesive so they can generally be machined immediately after they are pulled from the press.
Don't leave a panel in the press longer than the glue specifies. For some veneer adhesives this can cause the glue to get "globby" which then causes ripples and bubbles to form in the veneer. Leaving panels in for too long can also allow mold to grow. I've seen cherry and maple develop patches of mold in less than 6 hours.
Panel warping can be prevented by allowing both sides of the panel to dry evenly. Small light-weight panels can simply be hung by spring clamps from a rope or dowel. Large panels can be placed on a flat surface with dowels or small diameter PVC pipe underneath to allow air flow which creates an even drying of both sides of the panel. For more information, be sure to check out the "Curing Glued Panels" page.
7: Trim the Panel Using the Right Tool
If the substrate was cut to the exact size before the veneer was applied then a flush cutting router bit is typically used to trim the edges. This works very well as long as the edge of the substrate is square, free of voids, and has no glue runs that have dripped down the sides.
Instead of cutting the substrate to the exact size, I prefer to cut my substrate panels one inch larger in both directions and then apply a veneer that is a half inch smaller than the substrate. I will typically line up one edge of the veneer with one edge of the substrate so that the veneer is inside the edge of the substrate by an eighth of an inch. Then I use blue painter's tape to hold it in place. Once the panel has cured, I use my table saw to do the final cutting of the panel. I start by cutting the side opposite from where I lined up the edge of the veneer slightly off the edge of the substrate. Then I go back to the other side and trim the panel to its final side.
Cutting veneer with a table saw requires a sharp blade. I prefer a saw blade designed to cross cut plywood but my standard blade typically works very well if I keep the blade high so that the cutting action of the teeth is on the downward movement of the blade. Warning: I don't recommend doing this because it is dangerous to have excess teeth above the part which is being cut.
8: Avoid Sanding Through the Veneer
It is every veneer user's nightmare to have a perfect panel ruined because the sander cut through the veneer. It's no less than instant disappointment when the glue surface becomes visible. Don't let a worry about sanding through a panel prevent you from veneering a project. I've been there and I've learned from it.
Do yourself a favor and make a couple of test panels using a softwood veneer like walnut or redwood and a hardwood veneer like maple or Karelian birch burl. These panels don't have to be large. Even an 8" x 10" sample works fine. Apply the glue to the substrate, press the panel, and let it cure.
Draw a dozen or more lines across the veneer surface with a pencil. Be sure the pencil lines go all the way out to to the edges of the panel since this is where many sand-through mistakes can happen. Then use your favorite tool to sand the panel. I use a random orbit sander typically starting with 120 grit. Watch the sanding lines disappear and note where they are removed easiest. You will likely find that the edges of the panel are where the pencil lines are removed with the least amount of effort.
Continue sanding the panel and trying various amounts of pressure on the tool to see how it affects the removal of the pencil markings. You would normally stop sanding when the pencil lines are removed and the panel is smooth, but in this case go ahead and keep sanding. The goal here is to sand down to the glue layer so that you get a feel for exactly how long you can sand before disaster strikes.
Consider making a few of these test panels and test mistakes and with that you will have gained invaluable knowledge that will help you avoid sanding through a project veneer panel.
I draw pencil lines on every veneered project in my shop as a reference for the softness of the veneer. If the line is difficult to remove, then I know I have a hard veneer which I will not easily sand through. I also use the pencil trick to help avoid sanding through if the veneer requires any grain filling or if the raw veneer required a patch piece. Sometimes I end up with some light pencil markings left on the veneer but these are easily removed with a little denatured alcohol on a paper towel.
Be sure to check out the Veneering FAQ for more information.
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