| SoloWoodworker |
Page last updated ©2008 Charles Plesums Austin Texas USA | |||
| Home | Why | Business | Woodworking | About us | Contact us |
There is no one perfect glue for all uses. Therefore when choosing a glue, you should consider
There are a number of variants of PVA glue, but all are water based, and all are intended for gluing porous surfaces such as wood, paper, or cork. Since a previously glued surface is no longer porous (without total removal of the previous glue), a joint is not easily repaired. It is normally applied to both surfaces so that it can be worked into the pores. PVA likes to be clamped firmly, but not so tight that all the glue is squeezed out of the joint. PVA glue joints will creep slightly, an advantage when gluing dissimilar woods that may have uneven expansion/contraction, but that creep makes them undesirable for most bent-wood laminations. These glues will tack (so the joint no longer slips) in minutes, will set in an hour or so (the clamps can be removed if the joint is not under stress), but takes a day or more to reach full strength.
White or School glue is relatively strong, but is easily cleaned with warm water. In woodworking it is especially useful in veneering, since a the glue can be applied and dried, then the glue can be reactivated as the veneer is "ironed on" with a relatively cool iron. Unlike yellow glue, it can be reheated and re-adhered several times.
Yellow or Carpenter glue is stronger than white glue. It can be cleaned with water, but not as easily as white glue. Face/edge grain joints are generally stronger than the surrounding wood. It will fill a small gap, such as an imperfect joint, but is not a general gap-filler. Even though it does not claim to be water resistant, I have used this for cutting boards and turned bowls that are occasionally washed. If I have a mess after gluing a joint (a fingerprint, or ooze out that I missed), some clear water and elbow grease saves the day - even allowing the wood to be stained (unlike the water resistant versions of PVA glue.) When veneering by reactivating dry glue with heat, yellow glue requires a higher heat than white glue, and additional reheating after the first are not very strong. Elmer's Carpenter glue and Titebond ("original", I) are examples, along with many store brands.
Waterproof PVA glue, such as Titebond II, is water "resistant" but not waterproof. It cannot be used in marine or other applications that may be "soaked" in water, but is suitable for exterior trim. While it is still wet, it can be cleaned with water, but after it has dried (that glue fingerprint you missed) cleanup is extremely difficult due to water resistance. I do not use this glue... exterior furniture often gets into positions where water can pool at a joint, which could destroy the joint since it is water resistant but not waterproof. Titebond II passes the Type II ANSI water resistance test, which involves three cycles of soaking a joint for 4 hours, then baking in a warm oven for 19 hours - if the joint doesn't delaminate (fall apart), it passes. Since it doesn't meet my expectations for exterior use, and is harder to clean up in interior use, why bother? The furniture I make is not exposed to these conditions, and I probably won't be able to undo a joint if I need to fix it.
Titebond III claims to have an extended open time, and work in lower temperatures, than Titebond II. The bottle is labeled "waterproof" but the specification sheet indicates it should not be used where it can remain wet, so I consider it only water resistant, like Titebond II. Titebond III passes the Type I ANSI water resistance test, which involves boiling for 4 hours, baking at 145 degrees for 20 hours, then boiling for an additional 4 hours, and testing strength while still wet from boiling. I'm not interested, because the furniture I make isn't exposed to these conditions, and it means I probably cannot undo a joint if I need to fix it.
The open time of any of these PVA glues depends on when the water evaporates, allowing the other chemicals to react. Therefore you can extend the open time by moistening the wood or spraying on a mist of water after applying the glue, without buying a premium product. The lower operating temperature of some glues is only a few degrees - not significant.
Most water based glues are nominally cured in a day. However, the wood around the joint will have absorbed water from the glue, and will have expanded slightly. If you sand the joint, making it perfectly flat, in the first day or two after glue up, when the moisture level returns to normal that area will shrink, leaving a "sunken" joint. So wait as long as possible (multiple days or even a week or more) after gluing before further work. A stepped joint is similar - the parts of the joint end up uneven because the moisture content of the wood being joined was different before the glue up.
The big advantages of polyurethane glues such as Gorilla Glue are that they are waterproof, and can be used on both porous and non-porous materials. They are activated by minute quantities of water; often there is enough moisture in the wood to activate the glue, or a water mist can be sprayed on one of the surfaces. There is also enough moisture in the air to turn a container of glue to a solid block in months after it is opened. The glue is very strong, but it foams as it is activated, so it must be clamped to keep the joint together. The foam has virtually no strength, which is fine if you are gluing styrofoam (used under stucco), but the foaming glue has NO useful gap filling capability in woodworking... the joints must be good prior to gluing. If wood has been finished before assembly (the wood is no longer porous) polyurethane glue can be used where PVA cannot, but a joint failure is more likely to occur between the wood and the finish, than between the finish and the glue. The foam is relatively easily removed, and can be sanded and stained.
Polyurethane glue does contain a chemical that discolors skin - new skin with replace the discolored skin in about a week, but the discoloration cannot be washed out - wear disposable gloves!
Gorilla glue is highly advertised, but there are competing products from numerous other vendors. Be sure to also consider PL Premium construction adhesive, a polyurethane glue in calking gun tubes - it spreads well, but is thicker than the "normal" poly glues. It also seems to foam less than the other poly glues when I use it. The dispenser nozzle clogs after the first use or two, but I get many additional uses by punching a small hole in the side of the tube near the nozzle, and use the usual dispenser to pump out enough glue for small projects.
This is a very strong two part (resin and hardener) waterproof adhesive that can be mixed to cure within a few minutes, or with a different mixture or hardener can have a long open (working) time. In industrial variants it can be cured with heat or ultraviolet light. It can be applied in any thickness, including filling gaps (a major advantage doing repairs). It can be mixed with a filler like sawdust for even better gap filling. If it is warmed and allowed to soak into end-grain wood, that end grain can then be glued with epoxy (one of the few ways to get a strong bond in end grain).
Epoxy can be colored and opaque, but is often clear, even being used as a very hard waterproof finish on pens or a clear surface on tabletops. When used in boat repairs, it should be covered with a finish that provides protection from UV rays (sunlight). It is flexible, and very heat and chemical resistant (which also means it is difficult to clean up).
The "name brands" of epoxy include System 3 and West Systems (Marine), but there are also many other good products.
Veneering requires a slow setting glue that does not creep. There are a few pre-mixed glues, some two-part glues, but my favorite is urea formaldehyde, or plastic resin glue, often sold as a powder that is mixed with water a few minutes prior to use. The formaldehyde in the name scares some people away, but there is no smell that I can detect, and one report I read said the amount of formaldehyde our bodies create and exhale each day was far larger than the amount in that glue. It is then applied to one side (the substrate, not the veneer) with a notched trowel or roller.
Bent wood laminations have requirements similar to veneering...a glue that has a long open time (slow to tack) and does not creep. The glues used for veneering are also useful here. Some people use epoxy or hide glue, but contact cement and PVA glue are less than ideal.
This is a historically significant glue made from ... animal hides. It is still widely used in manufacturing (such as paper). For woodworking, it comes as crystals that are dissolved in water, and it has to be warmed (a glue pot or baby bottle warmer) for use. Like other animal products and food, it spoils if it is not used in days or weeks after it is mixed. The good news is the joints are repairable... if a chair support weakens, warm water will dissolve the old glue, and new glue can be added without problem. It was also historically used for veneering. It can also be sanded and stained. It does smell bad while it is being used (although it isn't a toxic chemical vapor).
A pre-mixed variant of hide glue is available, but doesn't work well for veneering, and doesn't seem as strong as "regular" hide glue.
Super glue is the most common trade name for Cyanoacrylic glue. For woodworking it comes in various thicknesses, often used for stabilizing or filling small flaws when turning, or as a solid coat for finishing. It can be used for joining non porous materials, but it too brittle to use for most furniture joints. It is infamous for gluing fingers to work pieces (consider having some "release agent" available for safety) but this same characteristic also makes it useful in surgery and emergency rooms as a modern substitute for stitches and staples.
Contact cement is applied to both surfaces to be joined. When it is practically dry, the two surfaces are brought together and immediately and permanently bond to each other. It has enough creep that the difference in expansion between the materials being joined is not likely to break the joint.
The art of using contact cement is how to bring the surfaces together perfectly the first time. It was widely used for plastic laminate (Formica) countertops, but other glues are better for most other woodworking applications. The solvent based contact cement is flammable, toxic, and smelly, and illegal in some jurisdictions, but seems to always perform well. The non-flammable contact cement has gotten various reviews from different users, from excellent to useless.
Aerosol cans of adhesive are available that do a great job attaching paper templates to work pieces to be cut out. These are often purchased at hobby stores, sewing stores, or office supply stores.
Commercial adhesives are available in spray form (the cans of glue look like gas tanks for cooking grills), and have set times measured in seconds (no clamps requires). They are not practical unless you are using many hundreds of dollars of adhesive per year.
Edge banding is often applied with a hot glue - in larger shops with a glue pot in the edge banding machine, or pre-applied to the edge banding material. (Glue pot machines can handle thicker and wider edge banding). Pre-glued edge banding can be applied with machines that cost from a few hundred to many thousands of dollars. For low volume work, an ordinary household iron can be used to melt the glue, followed by a block of wood to press the banding in place as the glue cools. If you must meet "green" standards, these glues almost always contain formaldehyde that will be emitted for the first month or two, and may destroy your meeting green standards.
Hobby glue guns melt and apply glue that comes in sticks. This is a useful temporary glue to hold templates in place, or to mount pieces on a lathe, but is not often useful in a final product.
This site (layout and contents) is ©2008 by Charles A. Plesums, 5702 Puccoon Cove, Austin Texas 78759-7177. ALL RIGHTS RESERVED. |