Bonding Agent Selection and Usage
by Randy Riley
Feb. 6, 2004

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Since I do diorama's, I currently use about 5-6 different bonding agents depending upon what the project needs are. Here's a short list of some of the ones I've used over the years:

Testors Styrene Glue (needle applicator)
Fast-acting CA Glue
Slow-acting CA Glue
Tenax with Touch and Flow needle applicator; or Squirt bottle applicator
Tamiya Glue
Elmer's White Glue
Duro Contact Cement
Two-part Epoxy
Gunze Mr. Surfacer 500 and 1000
Milliput
Aerosol Glue Scotch Photo Mount

Testors Styrene Glue
Maybe I'm old-fashioned but occasionally, I'll still use the old standard Testors styrene cement from the needle tip dispenser. Depending upon the type of plastic, you may need to select a different adhesive. Although the set-up and curing time is lengthy (up to 24 hours) compared to CA glue (5 minutes) or Tenax (1 minute), I find it easy to control the amount and placement of this glue on the model parts. The flow of the glue is slow and this allows me to take my time when placing pieces together and carefully inspect the fit as I mate the parts together. The bond from styrene cement is fairly strong when completely dry and will take a large amount of stress and abuse. The down side of using styrene cement is that the bond is as just about as strong as the plastic itself. If you change your mind about the fit of the parts later on - you're in a bad situation. The areas glued are permanently scarred if you do eventually manage to separate them. I have heard that placing the model in the freezer for a few days will allow the bond to release (I haven't had the occasion to try this though). Be careful not to spend too much time in an enclosed room with styrene glue - the fumes are harmful. Testors now has a variety of styrene cement which is less toxic. This is best for junior modelers since getting it on clothing and fingers does not represent a health problem.

CA Glue
The advent of cyano-acrylate (CA) glue (a.k.a. super glue) opened up a new chapter in model construction. The thin fast-acting variety allows you to put a kit together almost as fast as you can fit the parts together. This glue dries quickly and if that isn't fast enough, there is a "kicker" to speed up the process. CA glue is the only glue option to use for bonding the photo-etched details together with plastic. There are several down-sides to CA though. The quick flow of thin CA glue can create difficulty for you since it is difficult to regulate the amount applied. The fast set-up time can cause problems if you need working time for gluing large seams or surface areas. Regarding the bonding strength, I have found that CA glue can be just as strong as styrene cement, provided the fit of the parts is fairly tight. If you don't have a tight fit, don't expect super glue to bond effectively. A word of caution - don't use CA glue for clear plastic parts. The fumes alone will cause the surface to haze over.

The problem with using CA glue is that using it with the plastic holed applicators and tubes that come with it can result in a huge mess. The tip of the nozzle and applicators harden over after the first few uses and will make it impossible to get the glue out in the desired amount. To solve this problem I use a special metal hook applicator from Squadron which collects a single drop at the tip. Now, I don't even cut off the tip of the bottle nozzle - I unscrew the bottle cap and dip the metal applicator directly into the bottle. This makes the application much easier and I don't waste a lot of time keeping the tip unclogged. I find that a bottle of CA glue lasts quite a long time now. After an accumulation of glue builds up on the tip of the metal applicator, I just scrape it off with my model knife blade. I have even modified the metal applicator by carefully filing with my Dremel grinding bit down to a smaller size. Now it holds a small drop of glue, although placing it in difficult to reach areas is challenging. I have even glued special stands on the bottoms of the bottles to keep them from accidentally tipping over. You can also get the MicroBrush brand of plastic applicators, however these will eventually gum up with excess glue and have to be thrown out.

CA glue comes in no less than about four thicknesses depending upon the brand. The thicker variety is great for gap filling and will also provide you with slower set-up times. A must for all CA glue users is the debonding agent. This will dissolve the super glue, but requires a little persistence to work properly. A gummy residue forms after the debonding action occurs and may need to be wiped from the parts after the debonder works. I have found that you can easily remove 100% of CA from metal parts though. For some joints where you anticipate that more strength will be needed, you can reinforce the joint by sprinkling on baking soda at the joint before the super glue dries. Add more glue and baking soda as needed until you are happy with the results. As a reminder, CA glue fumes are also hazardous and it will bond anything - including skin...

Tenax Solvent
Tenax is a very strong styrene plastic solvent. It is super thin and will run readily into cracks - even more so than CA glue. Tenax actually dissolves the styrene when it joins it together. The bond is as good as or better than Testors Styrene glue. The model parts need to be tight fitting to draw the solvent in to the space between them. It doesn't harden the same way CA glue does when it's exposed to air. It actually evaporates when exposed to air. To apply it, I use a different applicator called the "Touch -n- Flow". This is a very fragile glass pipette tube with a needle tip. To use the Touch and Flow applicator, just set it inside the Tenax bottle and capillary action of the tube will draw it right up. You may want to tap the applicator gently to encourage quick refilling action. With a smaller bottle, the needle will not hold much glue so you'll find you'll be dipping the needle into the bottle quite a bit. Using Tenax this way, you'll want to hold the styrene parts together with a clamp or something to ensure that they don't shift while the solvent acts on the two parts. The bond sets up fairly quickly and is as strong as the original plastic. To apply it with the applicator, make sure you don't touch the needle into the join area for too long, or the solvent will totally run out onto the plastic. Just draw it quickly across the joint and hold it tightly for 30 seconds or so.

When using Tenax, you'll want to work quickly and reseal the bottle when you're done, since it does evaporate in the air. The fumes are hazardous and you shouldn't use it while smoking. You can buy the large bottles of Tenax. You may want to continually refill a smaller narrow bottle to minimize the evaporative effect. After using the needle applicator, it may get clogged from the melting action of the plastic. Keeping the flame away from the bottle (close it up) - simply take a lighter and hold it at the tip and viola! Problem solved. Don't overheat the tip as it will melt.

Tamiya Glue
This glue is a fast acting solvent similar to Tenax. There are few varieties with different thicknesses. I use the thicker variety which has a brush-like applicator joined into the cap. It's consistency is similar to the Testor's in the needle dispenser bottle, but it has a fast (15 minute) setting time. You may find it helpful to apply a thin layer of glue to both parts which will be joined. This will help them to bond completely and strongly. Be careful though, since it is somewhat thick the glue has a tendency to be applied thick - so only apply a small amount. This glue has a very strong odor which I suspect is not good to breath - so use in a well ventilated area.

White Glue
Regular White glue (Elmer's or PVA) is also useful in many circumstances. White glue will not "haze" (melt or alter) the surface of the plastic and is great for airplane canopies. It is easy to apply, dries clear, and can be trimmed with a sharp blade when dry. The drawback to white glue is that it does not have an extremely strong bond for plastic applications. For a strong bond, you need to join porous materials (like wood). Unless I am using wood, I only use it for joints where little to no strength is required between the two materials. I use Elmer's quite a bit in making diorama scenes. I mix it with fine dirt to make groundwork. It can even be mixed with Tamiya acrylic paint to give it any color you may desire. The best thing about white glue is that most brands are non-toxic.

Contact Cement
Yes, I said contact cement… I use this in diorama construction where dissimilar materials (i.e. wood and plastic) require really strong bonding. It is very flammable and very hazardous to breath - so keep use in a well ventilated area. The challenge with contact cement is the long set-up time. A fair amount of evenly applied pressure is required to get a good bond. Clamps are highly suggested since you'll have to let the glue harden 24 hours or so (depending how much you apply).

To apply it, you need to put it on the contact area of both joining surfaces. Then let it cure for 10 to 15 minutes. Then join the surfaces together and apply your clamps. The wait between the application time and the joining time is important.

Two Part Epoxy
Epoxy can be used in places where a bond is needed and the resulting seam will be sanded and filed. I use two-part Epoxy much less now than I did a few years ago. The quality and fit of the newer models has all but eliminated my need for it. As a matter of fact, I'm out of epoxy and I don't plan to get any more anytime soon. To use it, you mix the two parts equally together very well and then apply it. It will bond anything - so be prepared to have it stay a while. The set-up time for epoxy is also very lengthy (up to 24 hours) and requires steady even pressure to bond properly. But the finished joint is strong and can be worked extensively. If you plan to sand the joint after using epoxy, you'll probably sand away more plastic than epoxy - so use it very sparingly. Once it fully cures - it is rock hard.

I learned a trick a few years ago to wipe up excessively applied epoxy. Take a Q-tip and soak it in saliva for a few seconds. Then run the q-tip over the area removing the wet epoxy. It works like a charm. Don't use plain water as that will affect the bond properties.

Squadron Putty (White or Green)
There are several varieties of putty on the market and I have a couple of them for use in different applications. The one part white putty from Squadron is good for filling gaps and can more easily be sanded than epoxy. Squadron putty requires careful application since it will readily bond with surrounding areas as well as the desired application area. I find putty very messy and difficult to apply to small areas like cracks and seams. Masking is absolutely a must whenever you work with this stuff. I have found that putty doesn't add a lot of strength to the joint - so don't use it in areas that will be flexed. The fumes are hazardous so be careful with it. I don't use the Squadron much anymore as I find there are better ways to eliminate those seams and cracks. To make a thinner mixture of putty, pour a little acetone (nail polish remover) in a disposable cup and mix it well. This helps it spread a little easier.

Mr Surfacer 500 and 1000
I have yet to be able to interpret the instructions for this stuff - the writing is all in Japanese. What I have learned about MS has come from others on the Internet. Basically, MS is a thin variety of putty. It can be used to add texture to smooth plastic surfaces or to fill cracks. There are two varieties: 500 and 1000. This relates to their thickness, like motor oil. The 500 variety can be more easily sanded with a large grit sand-paper, while the 1000, a smaller grit. If you want to make the MS thinner, you can cut it with acetone (nail polish remover). A common use of the 1000 variety is for armor modelers to replicate the cast iron texture of Sherman tank turrets.

Milliput
Milliput modeling putty comes in different "grains" or fineness and comes in two-parts which have to be mixed. You'll notice that there is a large price differential in the different grains. The finer grains cost as much as 50% more. To use Milliput, take equal amounts of both and roll each into a thin "line" using your hands wet with water. Then wrap the two "lines" together like a brad. Then roll flat into a single line. Cut the line in half and repeat the process several times. Unless it is mixed well, it won't harden equally. After you've mixed it well, wash your hands thoroughly. Then mold it into the shape that you desire. Milliput does have bonding properties. It will join things together but does not have a large strength factor. Most modelers use Milliput make sand-bags or fill gaps in modified figures. I've even seen some modelers make their own figures from scratch with it.

Aerosol Glue (Scotch PhotoMount)
I occasionally use Aerosol glue in certain circumstances. Primarily I use it in diorama work where I'm putting leaves on one of my custom trees or on the ground. The difficult thing about Aerosol glue is controlling the application - it really goes everywhere. You'll probably want to mask everything that you don't want covered and even then use it very sparingly. Also pay attention to the overspray - make sure you use it outside where it won't wind up on a wall or on the carpet. Photo-Mount is a similar aerosol glue but it doesn't bond quite as permanently as this brand.

Conclusion
The important thing is to realize that you have options when bonding parts together. Be careful about applying more than one type of glue to the same area - mixing of some of these solvents can be disastrous to the plastic as well as be dangerous to inhale.