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Adhesives for Working Wood

A Primer


This article presents basic information about three different types of glue used by woodturners, namely wood glue, cyanoacrylate (CA), and epoxy.



What is a “gap-filling” glue?


Quite simply it is a glue that will bridge, or fill, gaps between two surfaces and produce a glue joint that is not significantly weakened by the gaps. Not all glues used in woodworking have this property. Epoxy and medium or thick CA are gap filling; thin CA and the “wood glues” are not.


Clamping glue joints, in general:


While epoxy and medium or thick CA do not require clamping, I think it never hurts to clamp a glue joint with moderate pressure. In many cases you can use the tailstock of the lathe to supply clamping pressure, such as when installing waste blocks on turning blanks for bowls or platters.


Wood Glue


This rather broad category includes white glue, yellow glue, and variations that offer somewhat different properties. The properties usually differentiated among them are (1) working time, which is the time the glue joint can be manipulated without affecting the strength of the joint; (2) the “initial tack,” which refers to the grip that occurs when the two pieces make initial contact; and (3) resistance to water, i.e., not waterproof, water resistant, or waterproof.  


Almost all such glues produce glue joints that are stronger than the wood to which they are applied. The wood will fail before the glue-wood interface does. Therefore, it is not really meaningful or beneficial to describe one as being stronger than another.


White glues are usually based on polyvinyl acetate (PVA). Yellow glues, sometimes referred to as Carpenter’s glue, are chemically very similar and are described as being “aliphatic resin” glues. Perhaps the best known brand of white glue is Elmer’s. A popular yellow glue is Titebond Original. There is also Titebond II, which is more resistant to water, and Titebond III which is said to be waterproof.


None of these are gap filling, and all require clamping pressure while the glue is setting up. Also, none are effective in gluing end grain to end grain, or end grain to face grain.


The general concensus is that clamps may be removed after 30 minutes or so, but it takes a full 24 hours for the joint to develop its maximum strength.


With that being said, years ago I did a test with kiln-dried spruce. Using ordinary yellow glue, I clamped a test joint for 10 minutes and then removed the clamps. When soon thereafter I broke the joint apart, the wood failed instead of the glue joint. The glue joint may not have reached its ultimate strength, but it was strong enough. However, for a different wood under different conditions, the results may not be the same.


A joint that is under stress is a whole different matter. For example, if you are trying to glue two boards together whose edges do not fit together perfectly but which can be pulled together with moderate clamping pressure, you should leave the clamps on for at least 12 hours and preferably 24.


Applications


Flat woodworkers use wood glue in almost every way imaginable, from gluing up flat panels to making joints of all kinds. Using the glue is part of their stock in trade.


However, woodturners don’t use it all that much unless they’re gluing up blanks from boards or making jigs and fixtures. Some use it to attach waste blocks, but most will use epoxy or medium CA instead.


Then there are the segmented turners who take big pieces of wood, cut them up into funny-shaped pieces, and then glue them back together. Without the glue, they would have to resort to turning solid wood.


The internet is loaded with tons of articles and videos relating to working wood and using various adhesives. Go there and you can learn more than you ever wanted to know.


Precautions


I don’t know of any health issues associated with wood glue. It is nontoxic and water soluable; it does not give off fumes. If you get it on your hands, wash it off. It’s about that simple.


Cyanoacrylate (CA) glue


This adhesive, known originally as superglue, is widely used. It sets up very quickly and requires no clamping. It is available in several viscosities ranging from ultra thin to a gel. The “thin CA” and “medium CA” are the most frequently used.  


The time required for the glue to set up varies with the viscosity. Thin CA will set up almost instantly; a rule of thumb for medium CA is 10 to 15 seconds. Thick CA should be given longer, perhaps a minute or two, before putting stress on the joint.


Small bottles of spray-on accelerator are available that will cause medium or thick CA to set up almost instantly. You apply the glue to one surface and spray the other with accelerator. When you put the two together, they seize instantly. Once they touch, there is no opportunity to adjust the position of one relative to the other.


I routinely clamp pieces that I’m joining with medium or thick CA (without accelerator). I think the clamping pressure helps to distribute the glue more evenly. In any event, the pressure must be applied only for a minute or so.


An item you should purchase along with the CA is a small bottle of “debonder.” This is a liquid that will dissovle CA glue inadvertently applied to human skin. And trust me, if you use CA, you will find the need for the debonder.


Acetone will also dissolve CA but does not work as rapidly as the debonder. (Acetone is very flammable, more so than gasoline.)


Applications


Thin CA is as thin as water and will easily penetrate and bond a hairline crack in a turning blank. However, if the crack has an observable width, the CA will flow into the crack but will not bond one side to the other. Thin CA is not gap-filling. Such a crack requires either medium or thick CA, but it is more difficult to get the thicker material to flow into the crack.


Because it has such a low viscosity, thin CA can be used to firm up small regions of a blank that are punky and too soft to work with normally. Simply apply the CA to the surface and it will soak in and do the job. However, it is likely that a lot of fumes will be produced as you do this, and the fumes are bad. You must avoid breathing the fumes.  


Along these same lines, thin CA is often used to strengthen the bark of a natural edge bowl when it appears that the bark is likely to break off.


Many turners use thin CA and sawdust to fill an undesirable void in a piece. Pack the void with sawdust and then drip in the CA. Instead of sawdust, crushed tourquoise or powdered copper or brass can be used to add a decorative effect.


It is possible to use medium or thick CA to attach waste blocks (or glue blocks) to a workpiece so that a tenon can be formed on the block. My preference is to use epoxy for this application. The epoxy takes longer to mix and apply, and you have to let it cure, but I have more confidence in the epoxy than the CA. Even so, many turners use CA sucessfully.


When cutting threads in wood with a tap, the threads are often fragile and may break off or simply crumble, depending mostly on the quality of wood being threaded. You can strengthen the threads once they are cut by wetting them with thin CA. Then, after the CA has been given time to cure, run the tap through again to clean up the threads.


However, give the CA at least 15 minutes to cure even though you are using the thin, instant CA. If you are too quick to run the tap through for the second time, the uncured CA at the bottom of the threads will make a mess of your tap, and the threads may be damaged as well.



Precautions


What does a bandsaw, a table saw, and CA glue have in common? Any one of them can send you to the emergency room in the twinkling of an eye. Although not as dramatic as loosing a finger to a saw blade, a tiny drop of CA slung or splattered into your eye will be very painful and will require prompt medical attention.


Suppose you fill a crack in a bowl blank with gap-filling medium or thick CA. To make progress rapidly, you squirt the crack with accelerator and note that the surface glazes over as the CA hardens.


Herein lies a pitfall. The accelerator will not reach the CA down deep in the crack and it will not be hard; it will still be liquid, just like the CA that remains in the bottle.


If you proceed to turn the blank and turn away the hardened surface of the CA, the liquid CA underneath will be slung out, and CA will be flying everywhere. This is how you can get an eye full of CA, and this is yet another reason to wear a face shield. Face shield or not, it is always a good idea to wear eye protection when working with CA of any viscosity.


Thin CA has the remarkable ability to appear in places where you least expect it. Suppose you are wicking thin CA into a hairline crack that happens to go all the way through the blank. The CA can run all the way through and drip out the bottom.


You can easily glue your hand or fingers to the bottom of the blank, or if the CA happens to drip onto your pants, you can glue your pants to your leg. When this happens, it burns! You look down and see “smoke” coming from the hot spot. After the fire dies down, which seems to take forever, you must then get the debonder and gradually separate your pants from your leg.


Earlier I mentioned the fumes produced when thin CA is applied to punky wood or a sawdust filler. Many people are sensitive to these fumes and some people develop a sensitivity to them (and CA in general) after years of using CA without any adverse effects. You can develop a sensitivity over time. The thing to do is avoid direct contact with CA or its fumes as much as possible, from day one.


My suggestion is to work with CA in front of a fan where the fan will blow the fumes away from you. And be reasonable. Don’t apply a huge quantity of CA to a big section of a blank all at one time. CA glue is nasty stuff.



Epoxy


This versatile adhesive comes in two parts, a resin and a hardener, that have to be mixed immediately prior to use. The most common are mixed with equal parts of resin and hardener. It has excellent gap-filling properties and does not require clamping.


Epoxies are available as “fast cure” or “slow cure,” which refers, roughly, to the amount of time it can be manipulated after the resin and hardener are mixed. The fast cure typically gives about three minutes working time (pot life) and is known as “5-minute epoxy.”  The slow cure is available in 15-, 30-, and 60-minute formulations and allows correspondingly longer working times.


The time to cure is much longer than the pot life. For example, I give 5-minute epoxy at least 15 minutes to cure before I stress the joint in any manner. If I’m attaching a waste block to a blank, I’ll give it at least an hour before I trust the glue joint to hold the blank under the stress of turning. A 30-minute epoxy should be given at least a couple of hours to cure.  


Denatured alcohol can be used to clean up smears of epoxy or to remove it from items used for mixing and applying it to the workpiece.


Mixing epoxy


Many people are turned off by the fact that measured amounts of resin and hardener have to be mixed prior to use. This does not have to be a big issue.  


For small quantities, squeeze identical beads of hardener and resin onto a flat surface such as a plastic lid from a butter cup. Then mix the two beads together with a strip of wood and the epoxy will be ready to use. Use denatured alcohol and a swatch of paper towel to clean up the lid and the mixing stick so you can use them again.


For slightly greater quantities (a tablespoon or so), the small plastic mixing cups from a hobby shop are handy. Add equal amounts of resin and hardner to the cup, and then mix the two together in the same cup. I usually discard the cups after only one use rather than to try to clean them.


The same technique can be used for greater quantities, using larger mixing cups, of course. However, I don’t recommend trying to mix large quantites of 5-minute epoxy. Most of its pot life will be used up by the time you get it measured and mixed so you have little actual working time left.


If you plan to add a filler to the epoxy, such as sawdust, coffee grounds, or powdered metal, mix the epoxy and then add the filler.


As epoxy sets up and cures, it goes through at least three stages. When first mixed, it is a liquid and can be poured or spread over a surface easily. Then, as the working time is about to expire, it becomes stringy and will not spread evenly. A bit later it becomes gummy and rubbery and will no longer flow on a vertical surface. As it hardens further, it reaches a stage where it can be carved easily. Finally, in the final stages of curing, it can be sanded without clogging the abrasive.



Applications


Epoxy is not the best choice for general woodworking applications such as gluing up turning blanks or, say, gluing together boards to make a table top. Regular wood glue (PVA) is more convenient to use and remains more flexible than epoxy. The flexibility is desirable because it allows the wood to move without stressing the glue joint.


Because epoxy is gap filling, I think it is ideal for attaching waste blocks to turning blanks where the surfaces may not be perfectly flat.


Epoxy will stick to metal provided the metal is clean and free of oil or wax. Lightly sanding the metal will both roughen and clean the surface; just don’t wipe the surface with an oily finger after you sand it. Epoxy is ideal for securing bolts, screw heads, and so forth in wood, as is done when making screw chucks or threaded mandrels.


Undesirable voids and cavities in turnings can be filled with epoxy mixed with a filler such as coffee grounds, sawdust, crushed turquoise, or powdered metal. (See the article on this website, Coffee Grounds and Epoxy as a Filler.)


Suppose you have a turning tool with a flat tang that you want to secure in a wooden handle. You can drill a hole in the handle to accommodate the tang and then use a mixture of epoxy and sawdust to fill the void between the flat handle and the round hole.


Mix up a batch of (slow cure) epoxy and sawdust, pack the mixture into the hole, and then insert the tang. Just don’t wait too long before inserting the tang. If the epoxy begins to set up, you may find it impossible to force the tang into the mixture.  



Precautions


I only know of one instance where a person experienced significant health effects from using epoxy. That was a loss of sense of smell caused by breathing the dust generated by sanding turnings and other art pieces that contained large portions of epoxy. In this case, the exposure to the dust was extensive and spread over a long period of time (years).


Of course there are nuisance issues: if you get the mixture on your clothes and allow it to cure, it absolutely will not come out. And drips of epoxy on your work surface or on the banjo of your lathe will have to be mechanically removed by sanding or scraping.



Problems with Epoxy


The most common problem that I’ve heard about from other turners is that the epoxy does not cure to a hard, sandable material. I think there are two possible causes for this. One is that unequal amounts of resin and hardener where used in the mix. To avoid this possibility, measure carefully, or if you’re using the bead method, try to squeeze out identical beads. Be aware, however, that the first bead will gradually flow out and get wider as you are squeezing the second bead. Take this into account.


The other possibility is that you may be using an inferior epoxy. They are not all the same. I suggest buying epoxy from a hobby shop that caters to the model airplane crowd or from a woodworking supply store. I do not buy epoxy or CA glue from the local hardware or big box store.


If you add a coloring agent to the epoxy, you should do a test to make sure the coloring agent does not alter the properties of the epoxy after it has cured. Adding sawdust or coffee grounds to epoxy actually makes it easier to sand.


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