Guide to Glue

There are myriad options on the market, we will focus just on two main glue types that we consider the most viable choices: cyanoacrylate and PVA wood glue.

Common Rules of Thumb

For a strong bond, the mating surfaces should be as clean and level as possible. Sand the surfaces down if necessary (but clean the sanding dust afterward), remove grease marks, paint, putty, and residues of previous gluing attempts.

Proper ventilation is an essential piece of advice that goes with everything, but it’s really twice as important with glues, which are often irritant or outright toxic and/or with slight narcotic effects.

Before you choose the right glue, think twice about all the aspects of the application. Size, the material of your model (PLA or Resin), and their intended use. Do you need a truly permanent bond or disassemble the model later? Are you going to post-process the seams somehow?

Cyanoacrylates

Cyanoacrylate glues, mostly referred to as CA glues or superglues (further only “CA”), are based on ethyl cyanoacrylate and related chemicals. The polymerization process is quickly started by contact with water. Normal air moisture is usually more than enough for a rapid reaction. If you want to accelerate it further, you can use commercially available solvent or water-based accelerators (activators), or just regular water in a spray bottle. A good trick is to breathe on the glued joint with your mouth (just make sure you don’t inhale the CA fumes) and let the breath moisture do the work.

However, usually, it’s rather the quickness of setting that poses a problem, especially with large parts. You have only very little time to adjust the joint, as it becomes sticky very quickly. This is twice challenging with resin parts, where the bond is almost instant.

Messing with the adjustment also compromises the final strength of the bond. If you don’t reach a correct fit in a few seconds, it’s usually better to clean off all the glue from the mating surfaces and start anew.

Suitability for PLA and Resin

CA is equally suitable for PLA and resin and it interacts minimally with the glued material itself. When drying, CA glue shrinks slightly and thick layers become coarse and wrinkled, so it’s not very aesthetic. Also, fumes from a larger amount of CA glue create white frosting (blooming) around the joint. The effect is most prominent when the fumes cannot escape (say, when you place a freshly glued object into a plastic container too soon). The longer the drying time, the more frosting. To minimize it, you need strong ventilation.

Curing

A regular CA is a rock hard when cured. Even though it dries almost instantly and is pretty hard right away, it reaches the maximum hardness only after several (8-24) hours. The hardness is a mixed bag though. CA has very high tensile strength, but quite low torsional and shear strength. That means the parts are much easier twisted or peeled than pulled apart.

Finishing

CA might be hard to sand down, especially if the surrounding material is much softer. This is not an issue with PLA, which is very hard itself but sanding for example glued resin parts can be tricky. The softer material yields faster, so you might end up with a patch of CA still present and shallow “valleys” in the material around it. It helps to sand the CA while it’s still not cured completely. On the other hand, if you are patient, CA can be sanded to a very smooth, glass-like surface. It almost doesn’t peel or crumble and remains solid even in very thin layers.

Storing

CA has a limited shelf life, it thickens and finally hardens rock solid. Usually, it’s a question of mere months. Obviously, moisture speeds up the process. It is always better to not use the container directly, but carefully put a puddle of glue on a clean, non-porous surface and work from there, using for example a cocktail stick.

Debonders

Debonder is a special product used for dissolving cured CA. While CA is often used in a household by the general public, the existence of debonders seems to be a well-kept secret known only to some hobbyists. That’s a pity because the “reversibility” they provide to CA presents a big advantage. You can dismantle a glued model with little or no damage, clean off any glue stains, etc. With a cotton swab soaked in debonder, you can wipe off the excess glue from a joint, thus save a lot of time otherwise spent sanding the surface.

There is a catch though. Commercially available debonders are based on various solvents and have an unpredictable level of strength and speed. Some do nearly nothing, some eat even through the plastic around. Always test a debonder reaction beforehand to prevent nasty surprises, even if you start a new bottle of the same brand (manufacturers sometimes change the formula without warning).

Disclaimer: Retro Replica Models accepts no responsibility for model damage caused by use of debonders.

Which Cyanoacrylate to Choose?

In general, price and quality do matter, to an extent. Usually, you can settle for something in between. Large companies like Henkel (Loctite) or Alteco make tubes which offer an excellent value.

There are many cyanoacrylate variants created by mixing the common basic substance with various additives, yielding very different characteristics.

Here are some examples:

• Gel (viscous) CA glues, mixed with ingredients like fine silica powder. They take a little longer to set, therefore are easier to use especially if you need to quickly adjust the position of parts. Their bond is slightly weaker. Since they dry longer, they usually produce more frosting. Are very handy for applications, where the bond is otherwise too quick, like with resin parts!

• Odourless CA glues’ advantage is not only their (near) absence of smell, they also don’t create the typical white frosting caused by the condensed fumes of a regular CA. That makes them suitable for example for transparent parts (e.g. light lenses). Their setting time is usually slightly longer and the bond is weaker.

• Extra-thin CA glues, with a watery consistency, similar to extra-thin solvent glues, used for gluing while employing capillary effect.

PVA/wood glues

A white glue that will bond dissimilar materials where there is limited stress. PVA glue doesn't have the strongest grip on plastics. Its key characteristics are water cleanup while drying, clear drying, and flexibility (It can be used for small parts where more time is needed for positioning).

It is very good for attaching clear parts such as light glasses and small parts such as badges. Recommended by Retro Replica Models as the best type for clear parts and small details.