A Sticky Subject…

As with the Marking Knife article elsewhere in this blog, this article is an updated version of one published in the Narrow Gauge & Industrial Railway Modelling Review back in July 1997 (Issue 31). It was one of a series of articles in my column At The Workbench. Recent questions and interest from online forums have convinced me to republish the article here. The illustrations and some of the information have been updated as needed.

In this article, I’ll examine the advantages and disadvantages of some of the more popular glues in use today for modelwork and present the basic principles behind making successful glue joints. The majority of the article will focus on joining scale lumber, but I will also discuss gluing common plastics such as polystyrene. In addition I’ll present a gluing system for scale lumber that I believe is far superior to other hobby-market commercial glue products available today.

Joint Design

We should start by discussing some of the basic principles and examples of joint design, especially important when joining wood. It’s important to remember that a joint which relies solely on glue will be much weaker than a joint that derives its strength from glue in combination with a sound mechanical connection. One of the most problematic and common joints for wood is the basic butt joint.

The most important thing to remember when joining wood is that end grain has no glue strength. It is easy to understand this. End grain is in fact the end of the grain fibres and channels. Regardless of the type of glue being used or the type of wood being joined, the glue simply absorbs into the grain fibres and channels traveling away from the joint surfaces. What you end up with is no glue on the joint surface, hence no glue strength. The purpose of joint design is to overcome this inherent weakness is as proficient and economic a fashion as possible.

Face grain, on the contrary, yields maximum glue strength because the glue absorbs and wraps around the grain fibres, binding the fibres together. Because end grain has no glue strength, the basic butt joint (as shown in the above illustration) is inherently weak. A simple solution to this problem in modelwork is to pin the joint as shown above. An alternate and better method, providing the timbers being joined are of sufficient size and you have access to a precision miniature table saw, is to used the Dadoed Butt Joint design as shown in the illustration below. Using a precision table saw, such as the one manufactured by the Preac Tool Company, in combination with the appropriately sized blade and a sawing fixture, it’s possible to produce the joints illustrated in this article with repeatable accuracy. This has the added advantage of promoting precise and square assembly, such as you might need in structure, bridge or trestle framing.

Illustrations of a few other useful joints for modelbuilding are posted throughout the article. These are, by no means, the sum total of joints that can be used, but they are perhaps the most common and most readily applicable for scale modelling needs. I haven’t included any of the many variations of dovetail joints, finger joints, spline joints, mortice and tenon joints or biscuit joints as these, for the most part, are impractical in the miniature timber sizes most often used in scale models. It would be a singular accomplishment, if not an exercise in madness, to reproduce a 1:48 or similar scale version of a blind or even half-blind dovetail joint.

The half-lap joint is an exceptionally strong and neat joint as maximum glue strength is achieved by the adjoining face-grain surfaces. Once again, using a precision table saw and suitable fixture it is possible to accurately assemble framework that is square and to precise dimensions.

The scarf joint is used for joining timbers lengthwise to create a longer timber. Again it is a very strong joint as the adjoining surfaces are face grain. The angled scarf joint illustrated was popular with ship builders in the 16th through 19th centuries as a way of creating long timber assemblies, keels for instance. The illustration shows a simple version of that joint. There were also hooked variations and others with a central “locking” block.

Popular Glue Types

Fortunately for model builders there are many types of glues available today ranging from general purpose adhesives suitable for a wide range of materials and applications to adhesives formulated for a single purpose. There are two important elements we must consider before selecting any adhesive: the materials to be joined and the function of the joint. The failure of many glue joints is most often the result of ignoring one or the other or both of these two elements. Supplemental considerations such as setting time, curing time, adhesive appearance, resistance to moisture and chemicals are all secondary considerations. Understanding the materials to be joined is the primary consideration behind adhesive selection. Once we know what materials we are joining we will be able to narrow our adhesive selection to only those that are formulated for our particular materials. Understanding the function of the joint will help us determine the appropriate joint design and glue application to use in order to have a failure-free joint, which after all, is our primary objective. It is important to remember that the weakest joint is one that relies solely upon the strength of the adhesive.

Super Glues May Not Be So Super

Unfortunately one of the most popular adhesive types used by model builders today is that based on Cyanoacrylate. Often called “super glues” or “instant” adhesives, this type of product has limited use in the model shop as it is unsuited to many materials we often need to join. Most of us are aware, it also has the ability to “instantly” join skin to virtually any surface, which though a drawback, can be avoided by careful handling and is not among the primary reasons why this type of glue is unsuited for most modelling purposes. One primary concern is that these adhesives have limited archival properties. They tend to oxidise. The oxide produced evidently has corrosive properties. The adhesive in the joint deteriorates over time and ultimately the joint fails. This behaviour is particularly noticeable when this type of adhesive is used in combination with brass and some white metals. I have also witnessed rapid oxidation on some polystyrene plastics. In addition, though their tensile strength is good, these adhesives often have poor shear strength.

Not only are these glues unsuited to joining such prevalent modelling materials as brass, white metal and plastic because of their oxidising and archival properties, they are wholly unsuited for joining wood because they are formulated for joining non-porous materials. This type of adhesive is formulated for joining closely mated, non-porous surfaces. For the novice model builder, achieving joint precision can be difficult to accomplish. To address this situation in the market, the adhesive manufacturers introduced gap-filling versions of these glues, which in my opinion, do nothing more than encourage sloppy workmanship. The goal of this blog is to encourage the development of precision model-making skills and precision is no where more evident than in the accuracy or lack thereof of your joinery.

In spite of all the negative press I’m giving this type of adhesive, I do keep a bottle in the workshop. My particular preference is for Loctite Professional Super Glue. I rarely use it for actual model assembly, but use if often for the quick assembly of temporary jigs and fixtures used to carry out other model-building operations.

Adhesive Selection

The selection of the appropriate adhesive is determined primarily by the materials being joined. For low-temperature metals such as Cerrobend and similar alloys used to produce many of the castings in contemporary kits, I recommend using an expoxy-type adhesive, preferably of the quicker setting variety such as “Five-Minute” epoxy. Devcon makes a wide range of epoxy products. It is important to follow the manufacturer’s instructions regarding proper surface preparation prior to joining. This type of adhesive is also useful for joining porous materials to non-porous, such as wood to metal. For brass or nickel-silver I recommend soldering over any adhesive. However, if you are unable to solder, use an epoxy type adhesive. With all adhesives, it’s important to remember that excess glue doesn’t make a joint stronger, just sloppier. Always use adhesive sparingly following the manufacturer’s recommendations. I once used an epoxy product called JB Weld to perform an on-a-road-trip repair of a push-rod tube on a motorcycle engine. The tube had been misaligned and the action of the push-rod against it had worn a hole in the tube allowing oil to pour out in a rather dramatic fashion. I wrapped a screwdriver handle in cellophane, used that as a core in the tube and filled the hole in the tube’s side wall with JB weld. After curing overnight and some quick filing on the outside surface to remove excess epoxy, I put the tube back into place and the engine performed perfectly for over 300 miles until I was able to locate a replacement tube. When I removed the repaired tube, the epoxy showed no wear at all and the repair was still sound. I only mention this here to show that epoxies have many and varied uses.

Adhesive for Polystyrene

For joining polystyrene plastics I recommend using MEK (methy ethyl ketone). Slater’s markets this material, actually a powerful solvent, under the brand name “MEK-PAK.” You can also often find MEK at commercial painting suppliers and better hardware stores, usually in quart or gallon containers. MEK is a highly toxic and flammable solvent. Repeated and prolonged exposure can cause brain and nervous system damage. It is important to use this material in a well-ventilated area and avoid using it near any heat or spark source. When used as an adhesive in model work we generally use minute amounts, but even so, be certain to take all the necessary precautions.

MEK is very easy to use and requires only a soft fine brush. The joint should be assembled dry. With the polystyrene pieces held together, flow a small amount of the MEK into the joint with your fine brush. Capillary action wll draw the solvent into the joint, softening and literally welding the polystyrene together. Hold the joint in position for fifteen seconds or so and you are ready to proceed to the next assembly. Any excess solvent will evaporate almost immediately. Because it is a powerful solvent you need to be careful not to get any on outside or painted surfaces. It will immediately attack any lacquer-based paints. If you do splash any excess solvent on a polystyrene part, let it evaporate. Don’t try to wipe if off as you will damage the surface with any wiping action. With a little practice you will find this is by far the cleanest and quickest method to assemble polystyrene plastics.

A few MEK tips: store your MEK in a wide-based, small glass jar with a tight fitting cap or fit a small jar into a wood base to reduce the possibility of accidental spills. Do not leave your bottle uncapped. This solvent evaporates quickly and keeping it capped will limit your exposure to the harmful solvent vapours. If you make an error in assembly and need to remove or adjust a previously “welded” part, flow some additional MEK into the joint and carefully separate the pieces. MEK will soften previously cured joints.

Adhesives for Wood

There are many adhesive options available for joining wood. Probably the most popular adhesives are the PVA (polyvinyl acetate) types. Usually a white liquid, ready for use straight from the container, it dries clear in about an hour. It requires some pressure to insure proper adhesion and sets fully in about twenty-four hours. To work properly however, PVA does require a mechanically strong joint to start with (see above section on Joint Design). PVA adhesives can be diluted with water and as a result are susceptible to moisture. I generally don’t recommend PVA type adhesives for scale model assembly. PVA is most often used by modellers in a diluted form for “bonding” scenic materials such as dirt and rocks to scenic dioramas.

Many other wood glues are available today. Another popular form is colloquially known as “Carpenters’ Glue.” Light yellow in colour, it is an Aliphatic Resin or Polyaliphatic Resin Polymer glue. Titebond produces several versions marketed in this country. These adhesives tend to be stronger, faster setting and more water-resistant than the PVA variety. They are most useful for joining large timbers or benchwork in combination with a mechanically sound joint. Pressure needs to be applied during the curing process to assure maximum joint strength and to squeeze excess glue from the joint surfaces.

Ambroid/Acetone Glue Method

I believe the best procedure for joining scale lumber is the “Ambroid/Acetone Glue Method.” This process is easy to use, produces very clean results with a minimum amount of practice, parts are assembled dry, joints cure quickly allowing you to work continually without waiting for glue to dry and the glue joint is waterproof. It requires two products: Ambroid Liquid Cement and Acetone. Ambroid was developed around 1900 as a glue for canoe repair, both birch bark and canvas. Its name refers to its amber colour and the celluloid from which it is made. It is still used for this purpose by traditionalists. At some point in its lengthy history it was discovered by the model airplane crowd and it became a standby for wooden flying model airplane assembly. It is sold today in many hobby shops (and some Canoeist suppliers) for that purpose. It is now made from recycled cellulose with amber dye added.

Ambroid cement is particularly suited to the process below though any similar type of liquid balsa cement will work as long as it is soluble in Acetone. Acetone is a common solvent, readily available at builder’s supply and paint stores. Again a word of warning: Acetone is a highly flammable and toxic solvent. Use only with adequate ventilation and avoid using it near any heat or spark source.

You will need two small glass bottles with tight-fitting screw caps or stoppers. Clean paint bottles are ideal. Fill one bottle with straight Acetone, cap it securely and label it accordingly. Open the second bottle and fill it approximately 40% with Ambroid Liquid Cement squeezed from the tube. Now fill the remainder of the bottle with Acetone and mix the two together until you end up with a brushable liquid  near the consistency of lacquer paint. If the mixture seems too thick or begins to thicken as you are working with it, just add more Acetone.

Using the glue is a simple process to learn. First, be sure you have adequate ventilation. Open the bottles of mixed glue and Acetone only when you need to load up your applicator brushes. Do not leave them open. Apart from the safety considerations, the Acetone will evaporate and the glue mix will start to thicken. Using a clean, quality brush an appropriate size for the scale you are working in (I use a #0 brush for most of my work in 1:48), apply a thin coat of the glue mixture to both surfaces being joined within the joint area. You will notice that it absorbs almost immediately into the wood and will be dry to the touch in a matter of seconds. When the first coat is dry, apply a second coat as before and allow to dry. The glue may be applied over plain or stained wood, however, it will not penetrate a surface that is covered with a heavy coat of paint. Work as neatly as possible keeping the glue within the confines of the joint area. Don’t worry if some strays. It dries clear and can be cleaned up with Acetone is necessary.

When the glue has dried, hold the pieces together in their final position, and, using a clean appropriately sized brush, flood the joint area with one or more coats of Acetone. Hold the pieces together until the Acetone dries, only a matter of a few seconds. The Acetone is absorbed into the joint area, softens the dried glue on both surfaces and draws it into the wood across the joint surface effectively welding the two pieces together with no visible glue line. This method lends itself to speedy assembly. You can apply the glue mixture to several pieces in succession, then “weld” the pieces together one after the other. Repeat the process on another selection of pieces and so on. Should you make a mistake in assembly, simply apply Acetone to the erroneous joint and separate. You can also bond polystyrene to wood. Simply “weld” the styrene piece to its corresponding joint area on the wood piece that has been pre-treated with the glue mixture. That’s all there is to it — a simple, fast, strong and clean gluing process.

The Ambroid/Acetone method will also form a relatively strong butt joint in the smaller sizes of scale lumber. Other glues will fail repeatedly in this application as explained in the Joint Design section above. Simply apply the usual two coats of glue mixture to the face grain component of the joint and several coats to the end grain component. The end grain will absorb more of the glue as it travels down the grain fibres and channels. Then, as before, assemble the joint dry and weld the joint with Acetone. It’s still not nearly as good a solution to the butt joint problem in the absence of a mechanical connection, but it will work quite well for smaller timber joints that don’t carry any load or are subject to any strain.

Well this is by no means an exhaustive study of gluing methods and wood joint design, but hopefully it will provide you with some insights into how to assemble cleaner, more precise and stronger models.

Tags: Acetone, Ambroid Liquid Cement, JB Weld, joinery, Loctite Super Glue, MEK, Model-making, scale lumber, wood glue