A great variety of adhesives are available for modelers, and some work better with certain materials than others do. Certain glues are better suited to specific materials and structures within our various models. Balsa and plywood kits, fiberglass and composite aircraft and even the easy-to-assemble ARF kits all require different types of glue to complete their construction. Let's see what's out there!

GLUE TYPES

Glues are divided into two groups: single-part (you simply apply the adhesive directly from the bottle to the parts being glued) and two-part (two separate ingredients must be combined before the glue is applied).

Polyvinyl acetate (PVA), aliphatic resin, polyurethane, cyanoacrylate (CA) and epoxy are among the most popular, but there are, of course, many other types of structural adhesives available.

PVA and aliphatic resin have been around the longest. PVA (white glue) has been in use since the late '40s; Elmer's is the most common brand. Although it will bond any moisture-free, oil-free, porous materials, it has characteristics that make it less than ideal for RC modeling. PVA stays relatively soft and plastic, so structural joints secured with it will move over time. Its lack of moisture resistance makes it unsuitable for exposure to water. White glue is, however, a good adhesive for gluing small foam models together-if they won't be exposed to moisture.

Aliphatic resin (Titebond) was introduced in the early 1960s; Titebond-II is a more recent development. Aliphatic resin is good for almost anything we want to glue on a model. It's very water-resistant and sandable, and it has good gap-filling strength, fatigue- and shock-resistance.

It can be used to join small pieces of wood with little or no clamping pressure. Glue joints can be handled after 15 minutes, but they must be left overnight for the bond to reach full strength. After a reasonable shelf life of about two years (usually), it starts to thicken when it reaches the end of its useful life. Aliphatic resin has a relatively short working time-about 15 minutes-during which you can shift the parts into position. It can be thinned slightly and cleaned up with water, and it's the least expensive glue. Its only negative is that it dries from the evaporation of its water base and tends to shrink slightly (just as PVA does).

CYANOACRYLATE (CA)

CA glues are reactive monomers and are available in various viscosities: thick, medium and thin. They are compounds that form chemical links (that is, they polymerize) when spread or pressed into a thin film. The very small amount of moisture that's present on most construction surfaces acts as a catalyst for the adhesive to cure. When properly applied, CA forms a bond that is several times stronger than the materials that it joins. Too much moisture, however, will degrade the strength of the bond. The speed at which the bond is formed is the greatest advantage CA offers; you can build a model in hours instead of days and instantly repair damage at the field. When used with an accelerator (kicker,) thick CA can quickly fill in gaps caused by damaged or missing pieces. CA glues are also fuelproof.

CA USES

CA is great for general balsa and plywood parts construction. Because of its gap-filling properties, thick CA should be used on parts that don't fit together perfectly and when you join two different materials such as balsa and carbon fiber. Thin CA should be used on parts that fit tightly together but not to glue balsa sheeting together. The joint it forms is much harder than the surrounding balsa, and this makes it very difficult to sand the sheeting smooth; aliphatic resin would be best to use here.

CA STORAGE

CA's relatively long shelf life can be further extended if you store unopened bottles in a freezer or refrigerator. You should allow the glue to return to room temperature before you use it. Store open bottles in a cool dry place that isn't exposed to direct sunlight. Don't store your bottle next to your accelerator; over time, even the faintest of fumes will cause the CA to thicken and cure.

UNCLOGGING

To prevent clogs in the applicator tip, keep it away from any surface that has been sprayed with accelerator. When you finish using the glue, lightly tap the bottom of the bottle on the workbench to knock the glue in the applicator tip back down into the bottle. Give the bottle a little squeeze to blow air through the nozzle to clear it out completely, then wipe the tip clean with a cloth. If the tip does become clogged, don't use a metal pin to clear the tip. This will only push the clog down deeper into the bottle, and it will clog up again when you use the glue. Cut the clogged tip off the bottle, or use a thin, 1/16-inch bit to drill out the clog and form a new hole. A better way to prevent a clogged tip is to use an applicator tip extension. If it becomes clogged, you can remove the extension, clear it with a thin wire and then replace it on the bottle.

BONDING

To form a strong bond, the parts you are gluing need to fit together tightly. Smooth all rough spots on the mating surfaces and always hold the parts together as tightly as possible while you glue them. CA will hold objects together with considerable strength within seconds; however, it will take several hours for the bond to reach full strength. Although accelerator greatly shortens CA's cure time, the bond will be stronger if you allow it to cure naturally.

For superstrong CA glue joints, make sure that your parts are held tightly together, and allow the glue to penetrate deeply into the joint. For poorly fitting joints, a gap-filling CA is a better choice than several layers of thin CA. When in their cured state, thick CA glues are less brittle than thin CA is, and in general, "odorless" CA is about 10 to 15 percent weaker than regular CAs.

CA glue gives off strong vapors when it cures, so use it in a well-ventilated room that allows plenty of airflow over your building surface. Some modelers have developed allergic reactions or have become sensitive to CA vapors-including the "odorless" variety. CA readily bonds skin to just about everything, and even experienced modelers will glue their fingers to a model from time to time. That's why there is CA debonder.

EPOXY

Epoxy resin is a thick adhesive that's packaged in two parts that have to be mixed together before the adhesive will cure. Part A is the glue base, and Part B is the hardener (catalyst). Most epoxies require equal parts of A and B, but the ratio isn't super-critical. You can "eyeball" the two amounts, and the epoxy will still cure properly. There are other types of epoxy that must be mixed in unequal proportions (2:1 or 3:1 ratios) that require mixing cups to get the ratios correct. Being thick and very sticky, epoxy does not run as easily as CA adhesives do.

Two-part epoxy resin costs about a quarter of CA's price, so it's much more economical to use, especially for large areas. Epoxy (1:1 ratio) is very easy to use, and in general, allergic reactions to this adhesive are rare. Epoxies can be used to bond most materials together, but you must first roughen the surfaces a little. You can easily bond foam with epoxy, and a wide range of cure times-from 5 minutes to 4 hours-is available to give modelers plenty of time to align the parts that they bond.

MIXING

When you mix epoxy, any paper scratch pad or scrap piece of flat material can be used as a palette. A plastic coffee-can lid makes a great mixing surface because of its raised outer border, and as it's a flexible material, it is easy to peel cured epoxy off it. In cooler temperatures, if the epoxy becomes thick and difficult to pour out, you can microwave it (for about 10 seconds on high); warm epoxy flows more easily.

To mix epoxy, squeeze out equal lengths of parts A and B next to each other, and use a Popsicle stick or other piece of scrap to mix them together thoroughly. Roughen up the surfaces to be epoxied with 100- to 120-grit sandpaper to give the epoxy something to grip. If you want a thinner mixture to pour into a tight space, thin the epoxy 15 to 20 percent with isopropyl alcohol.

As with CA, slower-setting epoxy produces a stronger joint because it can penetrate deeper into the pores of the wood. As a rule, if you have a 5-, 15- and 30-minute epoxy, the 30-minute epoxy's bond will be the strongest when it's cured. However, an extra-slow-curing epoxy (2-hour, for example) isn't any stronger than 30-minute epoxy. It only gives you more working time before the adhesive thickens and cures.

For this reason, don't use 5- or 10-minute epoxy to glue wing halves together! It will cure before you cover all the mating surfaces. Take your time and use 30-minute epoxy!

Epoxy is excellent for laminating two or more pieces together, especially parts that are subject to high stress and vibration. Landing-gear blocks and firewalls are two main areas where epoxy is the best adhesive to use. Slow-setting epoxy is ideal for gluing the horizontal stabilizer and vertical fin to a fuselage. The slower curing epoxies allow you plenty of time to line up the tail feathers with the fuselage. The excess epoxy that oozes out of the joint is easy to clean off with paper towels and rubbing alcohol.

Epoxy's main disadvantage is its weight. The resin doesn't evaporate while it cures, so what you apply is what's left on the parts. Use it sparingly; extra epoxy adds only weight-not strength.

Some of the quicker-setting epoxies aren't resistant to long-term exposure to raw nitro fuel or water, and ambient temperatures also affect cure times. Cold weather lengthens the cure time; warmer weather will hasten the curing process. Always mix epoxy resin according to the instructions. If you add too much part B, it may never fully harden, or it may become extremely brittle and lose a great deal of its bonding strength.

FINAL THOUGHTS

Don't get stuck with the wrong glue! The more you know about the vast array of adhesives available the easier it will be for you to properly build your models. They'll last longer, and you'll get more fun out of our hobby.

Copyright Air Age Publishing Feb 2004