Introduction to the Process of Veneering:
Perhaps it is unfair, but woodworkers are often judged by their ability, or in-ability to use veneer. Many woodworkers avoid veneering thinking there is some sort of voodoo involved in the process, when in fact it is quite simple and will add a very important and necessary skill to your bag of tricks. A whole new world of expression, possibility, problem solving and design present themselves once veneering is part of your skill set.
Perhaps the most important benefit to veneering is the ability to work with exotic, precious and highly figured species of wood that are substantially more affordable than solid timber. In fact, many of the more specialized veneers are rarely available as solid wood. Slicing, rather than sawing up a precious log is a hundred times more cost effective. There is almost zero waste. Slicing veneer creates no kerf or sawdust. Far more profit is to be made by turning the planets greatest logs into veneer.
I once read that one saw kerf could represent five or six sheets of veneer or more. One board that measures 1" thick X 12" wide X 10' long is 10 board feet....or in this case also 10 square feet. This board required several saw cuts, possibly some thickness planing, and drying before arriving at the hardwood store. Once a woodworker gets the board, he or she would have to machine it again. If this lumber had been sliced into veneer, there could have been as much as 300 square feet of veneer and all of it would be perfectly sequence matched.
Slicing provides perfect mirror images between each sheet. When there is saw cut waste or kerf between faces, much of the definition and accuracy is lost.
Preparation:
Some veneer may require some level of preparation before application. When veneer is stored in an overly dry environment, it can become wavy or bubbled in appearance. If you attempt to flatten this veneer when dry, it will crack. Many highly figured veneers or burl veneers may need to be flattened prior to gluing. It is quite simple to flatten veneer.
To flatten veneer just mist both faces of the veneer with some water. Some folks use a very week solution of water mixed with some PVA glue, called a sizing, this helps seal up open grain and prevent glue from passing through the veneer.
The idea of the water is to soften the fiber of the wood and to make it pliable enough to be made flat once again.
Next, cut up a bundle of newspaper or newsprint to a size slightly larger than the veneer and lay a few sheets between each layer of veneer to be flattened. Stack all these up, add a platen to both the top and bottom of the stack, and clamp down with several C clamps.
Initially you will need to un-clamp, exchange the now damp paper with dry paper, and re-clamp, once an hour or so for most of a day. In the next day or two, depending on how hot and dry your shop is, you will continue to check on the paper and remove as it continues to absorb moisture until the paper appears to no longer do so. The veneer should now be perfectly flat and ready to work with.
Planning:
The next stage in the process is planning. In many cases you will need to add an edging to your substrate prior to the veneer being applied. If this is the case then be sure your edging is as straight and flat as the substrate. Make sure there are no gaps or voids. The veneer will dip or fall into these gaps and may be visible in the finished product.
A substrate can be many materials. MDF is the most widely used material these days. You can also use particle board, plywood or even solid wood.
Although veneer is available in many widths and lengths, you may not have material wide enough for the project panel. When using solid wood you can joint two edges square and straight then glue them together to make a larger panel. When veneer needs to be wider we also ‘joint’ two edges straight then 'tape' the seem together. Veneer can be jointed using an actual jointer, but the worker needs to sandwich the veneer between two flat solid wood platens to keep the edges square to the table and to keep the veneer from collapsing while running the sheets over the machine. The more common method of jointing the edges of veneer involves a simple shop fixture that secures one or more sheets of veneer between two permanent but vertically movable platens and a long hand-plane or jointer plane is used on its edge to straight line the edges of the veneer.
Once a two perfect mating edges have been achieved, we then use a special type of water based paper tape to 'stitch' the joint together. This tape is very thin paper with a water activated glue on one face. The tape is applied to the non-glued face. Starting at one end, a series of 3" long pieces of tape are used to pull the edges together over the entire length. Once this is completed a single length of tape is then applied over the full length of the seem or joint. These pieces of tape hold the joint closed while it is being pressed as well as prevents glue from squeezing through any gaps there may be in your joined edges. Sometimes the tape resists sticking to certain woods, just use a shop iron on low heat to get the tape to set perfectly. When I am taping seems I just iron all the tape every time.
The thinness of this tape comes into play during the pressing process. Thick tape would prevent even pressure over the entire panel. The water based glue and paper composition is removed after pressing by applying a very small amount of water to the tape to soften the glue and paper which will then peel and or scrape off quite easily. Any remaining residue will be removed during the sanding and finish prep process.
Glues:
This is often the area where the 'bad reputation' for veneer is generated. Contact cement is soft and pliable and reacts to both heat and moisture. Contact cement is for plastic laminate, not veneer. And to say it another way: Do not use contact cement to apply veneer.
(NOTE: In reality contact cement is a marginal choice even for plastic laminate, I personally prefer to glue on laminate, in marine applications I use epoxy or UF glue.)
One big consideration when gluing down veneer is the 'open time' of the glue. Open time refers to how much time you have to apply the glue to all the surfaces, get all your parts lined up, into clamps and under pressure. If your chosen glue begins to set before it is under pressure, the possibility of failure is certain.
Both yellow and white PVA glues are OK for veneering, but keep in mind that their open time is very short. I use these glues for small panels that will all go together very quickly, as long as open time is not an issue. But, you will have to allow for the access moisture inherent in these glues to dissipate.
The very best glue for any veneer application is urea formaldehyde based two part glue.
Advantages of UF glues:
- A longer open time.
- Can still be cleaned up with a damp cloth.
- Once set, the glue is waterproof and heat proof.
- UF glue dries very hard. In fact, this glue dries crystalline.
- Does not introduce moisture because of the chemical curing process.
Because of its hardness and heat resistance UF glue is easily sanded and will not soften through the heat of sanding. Once set there is no moisture issue. With PVA glues there will be a period of time required to allow all the moisture to evaporate.
The down side to this glue is that it is not fast, once under pressure the project will need to sit over night before being taking out of clamps. But, if you consider that you will need to wait for the moisture in PVA glues to evaporate, the speed becomes much less of an issue.
The mix ratio of powder hardener to liquid resin is usually around 15% by weight.
Pressing:
The simplified basic principal for gluing down a veneer to a substrate is to apply even pressure over the entire panel through 'pressing'.
The pressing process can range quite dramatically. From giant hydraulic heated presses, heated vacuum presses, vacuum bags, and cold screw presses all the way down to some shop made cauls and platens. These methods all work. Some methods are just much faster and more efficient for production situations.
The most basic method is the Caul and Platen technique. You just need a few clamps, some MDF for platens, and some MDF or solid wood for cauls.
A caul is also know as a bearer or strong back. In a press it is easy to apply pressure to the middle area of a glue-up. But, if you are restricted to using regular hand clamps or bar clamps it is not possible for the clamp heads to reach into the center area of a glue-up. Using shop-made cauls, with a very slight curve over the length, make it possible to apply even pressure right across a glue-up. Obtaining the correct amount of middle pressure is the key. There are a combination of factors that need to be considered when making a set of cauls. The thickness and section of the caul, the length and the material. All of these factors work together to determine the pressure the middle of your caul generates.
I often just use some scraps of 1" MDF for cauls. They are often around 2" wide with a steady 'rocker' or curve machined into them. The amount of curve varies, but if you shoot for about a 1/8" gap at either end of the caul and a nice smooth transition, you should be in the ballpark. My shop has a large supply of cauls meant for all sizes and pressings. They are useful for many years.
You are also going to need a set of 'stand-offs'. Stand-offs are kind of like a small pair of saw horses that you can use on top of your work bench. I make a simple pair of stand-offs by using plywood or MDF scraps and form a long 'T' shape that suits the size of glue-up I am doing. Be sure the stand-off height is such that the head or screw end of a bar or beam clamp can be 'down', clear the bench top and still be accessible.
Be sure these stand-offs are the same size, parallel, and made strong enough to support the weight of the entire glue-up....including all the clamps.
When prepping the veneer be sure to make the outside dimensions at least ¼" longer in all directions. You need the veneer to overhang in case it slips during the pressing and to be sure to have well bonded edges ready for eventual trimming.
When veneering any type of substrate you will have to veneer both faces. This is very important. By applying veneer to both surfaces you will create even and balanced stresses and ensure a flat product.
Also, keep in mind how you are going to trim or size this panel after pressing. In most cases you may want to have a slightly over-size substrate so as to have extra material for final trimming to size on a table or panel saw.
If you are gluing your veneer to a substrate with a solid wood edging and small veneer saw, sharp knife and a laminate trimmer will be most effective.
After removing the panel from the clamps etc, the overhanging edges are very fragile. Take care when handling the panel and take off the excess veneer as soon as possible...even if you are not cutting to final size right away.
Sanding is very quick on veneered panels. If everything went down as planned and there are no lumps or dents, there will be minimal sanding. You should be able to start at 150 or 120 grit. A stroke sander is a huge advantage for this kind of sander, but if you don't have access to one of these giant machines a well tuned hand belt sander with a 150 grit belt and a ½ pad sander or hand sanding block will make quick and accurate work of a veneered panel.
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