Adhesives

Adhesives

There are a variety of types of adhesives available. Each has a useful purpose as well as things it definitely

shouldn't be used for. This page is intended to outline the most common adhesives used in model-building

and give guidance on selecting and using the correct adhesive.



What you need to know about any glue you use:



 Strength:



As a rule of thumb, stronger glues tend to be heavier. Therefore, only select a glue that is

strong enough to do the job. For example, there is no good reason to use epoxy to glue

wing sheeting together but a lot of reasons not to. Slow drying glues tend to be stronger

than fast drying glues because they have more time to soak into the wood.



 Materials it can bond:



Most glues are intended for certain materials. Using the wrong glue can cause a variety

of problems including excess weight, difficult finishing and glue joints failing.



 Fuel-proof:



Fuel should not be able to get inside the airframe and fuel-proofness isn't much of a

consideration for general construction. Fuel tanks can and do split open from improper

assembly, defective molding or design or a crash. Therefore, the fuel compartment

should definitely be coated with something fuel proof such as epoxy or polyurethane

(paint). Any joints around the firewall should also be glued with a fuel-proof glue.



 Sanding ease:



How difficult the glue is to sand. Often you will need to sand a glue joint between two

pieces of wood. If the glue is significantly harder than the surrounding materials, the

glue will not sand away as quickly leaving a ridge that will be seen under the final

covering.



Terminology:



Adhesives are described by various properties. Most glues are of one of two types.



 Evaporation types



Glue is solvent or water based and dries by evaporation of the solvent.



 Chemical cure types



These glues harden by chemical process. They are further broken down to one or two-

part glues. Two part glues must be mixed in some ratio before the glue can be used.



Note: Any glue that needs to be mixed should be mixed in a non-porous container. If you try to mix epoxy

on something like a piece of cardboard, there is no way to ensure the glue will be mixed in the proper

proportions which may result in the glue not curing properly. I mix small quantities on a piece of glass or

on the bottom of an empty soda can. Excess glue can be wiped up with solvent while wet or scraped off

with a razor blade after it's cured.



One-part glues that cure include Cyanoacrylates (AKA Super Glue or CA). Two-part glues include

epoxies.



If you happen to say "dry" when you mean "cure" someone will undoubtedly correct you. Even though he's

right, feel free to ignore him. For all practical purposes, "dry" and "cure" mean the same thing.



Pot-Life:



How long the glue stays useable in the pot.



Working time:



This is not the same as pot-life. Glues that cure tend to heat up. In the

pot, they will cure faster than in a thin film. Therefore, many of these

glues can still be worked after being applied to a part even though the

glue in the pot is too thick to use.



Note: The time given for epoxies is the working time, not the curing

time. For example, 15 minute epoxy has a 15 minute working time.

Cure time is usually 20 -60 minutes depending on the brand and

climate.



Cure time:



How long a glue takes to fully harden. Note that the time given is for

practical purposes. Most glues that cure tend to continue the chemical

curing process for months.



Shelf-life:



All glues have a shelf life. This is how long it can sit on the shelf

before it goes bad. My advice is to not buy any more glue than you can

reasonably use within a year or so after you purchase it even though

some glues have shelf lives of years. Shelf life is strongly affected by

the climate (heat, humidity, UV light, etc.).



Set (also Tack or Grab):



When the glue "grabs" but not when it's fully dried or cured. For

solvent and water based glues this is when the glue reaches a state

where the parts are firmly held in place, but could be taken apart -

possibly without damaging anything. For adhesives that cure, it is the

stage where the glue has cured to the point where the parts are firmly

bonded in place, but has not fully hardened.









Techniques:

Double-Gluing:



Double-gluing is a technique that should be used whenever gluing end-

grain wood. End grain needs to soak up a certain amount of glue to

make a strong joint. If you apply glue and immediately put the part in

place, most of the glue will be forced out of the joint and the end grain

will soak up the glue that's left leaving very little glue in the joint.



What you should do is put glue on the end grain and let it soak in for a

minute or so. Fit another part while you're waiting. After the glue has

soaked into the grain apply a little more glue and put the part in place.



What I actually do is squeegee the glue back and forth for a minute or

so using a popsicle stick or toothpick to force the glue into the grain.



Another type of double-gluing is when using Solvent glues (Airplane

glue). These glues dry so fast that when you put glue on one piece, it is

dry by the time you join it to another piece. It actually isn't dry, but has

gelled. The answer to this problem is to add glue to both pieces. When

you join them, the solvent that is trapped under the gelled glue will

dissolve the gel long enough for the pieces to adhere to each other.



Tack Gluing



A part is tack-glued knowing that it will be broken or cut loose later

when there is no other practical way to hold the part in place

temporarily. Therefore, only the smallest amount of glue necessary to

hold the part is used.



This technique is usually used for shaping parts in place that you will

want to remove such as cowls carved from balsa or wing tip blocks that

will be hollowed after they are shaped on the airframe. Another

example is if you want to shape an aileron in place on the wing.

Clamps and pins are not feasible because they will be in the way when

it comes time to sand. The aileron is tack glued to the wing, shaped

and then cut loose.



Glues:



Aliphatic Resins (Carpenters Glue)



Fuel proof: No. They are fuel proof to a point, but extended exposure to raw fuel or exhaust residue will

break them down.



There are also so-called "sand able" aliphatic resins. I don't use them because whatever the additive is that

makes them sand able must make the glue weaker. Besides, normal carpenter's glues sand easily enough

that it's not really an issue.



Examples:



 Titebond

 Elmer's Carpenter glue

 Pica Gluit

Inexpensive and strong. I primarily use Carpenter's glue for my construction. It gives me

time to ensure everything is as it should be before the glue sets up. It also makes the

neatest glue joints because excess glue removes easily with a damp sponge or paper towel

while the glue is still wet.



Note: A round toothpick with the end cut at a bevel is a great little scooper to remove

excess glue that squeezes out the joint without smearing it everywhere.



Use for:



 General construction.



Don't use for:



 High-stress areas (firewalls, landing gears, etc.).

 Laminating broad areas - it will cause severe warping. If you

can place the part under a lot of weight for at least 24 hours,

then it can be used for laminating. Keep in mind that the glue

dries by evaporation and can take a long time to dry

completely.

 Edge joining sheets of balsa - it doesn't sand well and will

leave a nasty ridge when you try to sand the sheet flat.









Traditional Solvent Glue (Airplane Glue or tube glue):



Fuel proof: Yes. They are fuel proof to low nitro fuels. I don't know at what nitro content they are no

longer fuel proof.



Examples:



 Ambroid

 Sigment



This is the type of glue that comes in a tube and is used for wood models. It is fast-drying and lightweight.

The solvent in this glue has a noxious odor and it should only be used in a well ventilated area.



This type of glue is the best thing going for joining sheet wood together. It sands very easily and is more

than strong enough for the task. First, trim the two pieces of wood so they mate squarely. Then use a few

short pieces of masking tape to hold the pieces together. Only tape one side at first. Next, open the joint

and run a bead of glue down the joint. Place the wood with the masking tape side down on the board and

press it flat. Wipe up any glue that has squeezed from the joint and then tape this side and allow to dry.



Another use of this type of glue is to seal grain so that it doesn't fuzz when sanding. For example, I was

recently working on a part that had laminated edges that butted against a stick. The stick was radiused on

the end making it go from end grain to face grain. What happened was that I could never get the radius

smooth when sanding due to the different types of grain - the end grain kept "fuzzing."



To solve the problem, I put some Ambroid on the end grain and rubbed it in with my finger. I did this

twice more and then it was easy to finish sand using 400 paper. If you rub the glue in with your finger until

it's dry (about 20 seconds) you can sand immediately.

Use for:



 Stick and tissue models

 Edge joining sheet wood because it is very easy to sand.

 Gluing some types of plastic parts to wood models.

 Sealing dissimilar grains to make sanding easier.



Don't use for:



 General construction of models larger than 1/2A size.









Cyanoacrylate (CA) AKA "Super Glue"



Fuel proof: No. They are fuel-proof to 0% nitro fuel (FAI fuel), but nitro methane dissolves cyanoacryates.



Examples:



 Hot Stuff

 Zap



Any time there are a zillion tips in magazines on solving a problem, it means there is a fundamental

problem with the item that probably won't be solved. Clogged tips on CA bottles are a great example of

this kind of problem.



I generally avoid cyanoacrylate glues because they are expensive, make messy joints, skin from my fingers

almost always ends up on the structure somewhere, and the fumes are horrible. Additionally, I've noticed

that CA joints on some of my older models started to disintegrate by turning into some white, powdery

substance.



They are fast and strong, however. Nitro methane dissolves them - hence they are not nitro fuel proof.



Use for:



 Hardening threads cut into wood.

 Gluing fiberglass or carbon fiber (epoxy is preferred)

 Gluing difficult to clamp items when you don't want to hold

the part for the two hours another glue would take to dry.

 Use for some types of dissimilar joins such as carbon fiber to

wood.



Don't use for:



 High-stress areas.

 Areas exposed to raw fuel.

 Clear plastic parts - CA can fog the plastic.

 Foam - CA dissolves foam.

Epoxy:



Fuel-proof: Yes.



Strong but heavy and expensive. Epoxies are used for bonding high-stress areas as well as items that no

other adhesive will bond together. Additionally, epoxy can be used for applying fiberglass cloth and

making fiberglass parts. There are two kinds of epoxy resin that I know of. The most common type is used

as an adhesive. The second type is used for laminating and tends to be thin in consistency. There are also

various epoxy putties, etc. This section pertains only to the adhesive.



Use slow-drying epoxy (30-45 minute working time) whenever epoxy is called for. 5-minute epoxy is of

little use except in special circumstances. It is heavy and weak, doesn't cure properly and gets brittle with

age. You really shouldn't make field repairs which is a common use of 5-minute epoxy. Generally, epoxy

should only be used on load-bearing components. If the part that breaks isn't a load-bearing member, then

using epoxy adds unnecessary weight. If the part is a load-bearing member, then it's probably important

enough to get it repaired right that the repair should be made in your shop and not at the field.



The sort of "special circumstance" I would use 5-minute epoxy for is to create a smooth, non-porous

surface for servo tape. Place a sheet of waxed paper on a flat surface and apply a thin coat of epoxy to it.

Then put a piece of 1/32" plywood over the wet epoxy (approximately the same size as the servo. On top

of that put piece of waxed paper and a thicker piece of scrap wood as a clamp block and clamp the whole

thing down. Once it's cured, peel the plate from the waxed paper and trim off the excess epoxy. Locate the

plate in the fuselage and glue it in place with the cured epoxy side out. You know have a smooth, non-

porous surface that servo tape will stick to very well.



Use for:



 High-stress areas.

 Applying fiberglass cloth.

 Fuel proofing firewalls and fuel tank compartments.

 Bonding fiberglass and carbon fiber.

 Mix with micro balloons (microscopic glass beads) to make a

putty that can be formed into fillets and sanded easily. I also

use epoxy with micro-balloons to make perfect fitting wing

saddles.

 Bonding dissimilar items such as metal to wood.



Don't use for:



 General construction - it is too heavy.



Epoxy Additives:



Chopped fiberglass:



Mix with epoxy to strengthen joints. I mix it with micro-balloons and epoxy when I'm

building up wing fillets to strengthen the fillet.



Micro-balloons:



These are microscopic glass beads that look a lot like baking soda. They are generally

used with epoxy to create an easily sanded putty. The gotcha here is that when you use

them on balsa, some of the epoxy will soak out of the mix into the balsa and still create

areas that are difficult to sand. It's not a big problem, but just something to be aware of.



When I need something extremely easy to sand, I mix micro-balloons with tube glue

(Ambroid). It dries very fast so I only mix small amounts and use it immediately.

Working time is something like 30 seconds, so obviously it can't be used for big jobs.

Basically I use it to fill in small seams between sheets of balsa or similar tasks. It can be

sanded after about 10 minutes.









Polyurethane glue:



Fuel-proof: I believe they are, but I really don't know.



Examples:



 Gorilla glue

 Elmer's Polyurethane glue



I've just started using this glue so I can't say much about it. What I do know is that I won't use it for general

construction because it is messy and expands out of the joints. As far as I know there is no solvent that can

clean this stuff up.



I've been using Gorilla glue to laminate parts in my current project. It seems to be working very well and is

much quicker than my usual epoxy method. Since this is the first time I've used the glue, I don't know how

well it will hold up, but so far it looks like a winner.



I've read that these glues have a short shelf-life. Unfortunately, I purchased the larger bottle of Gorilla glue

and since I live in a hot, humid climate, there is a good possibility that the glue will go bad before I use it

all. For that reason, I'm looking for excuses to use it so it doesn't go to waste.









Contact cement:



Fuel-proof: Unsure.



Heavy and has very limited uses in model-building. Doesn't tend to warp sheeting badly enough to cause

problems.



Contact cement is applied to both surfaces and allowed to dry to the touch. Then the two parts are brought

into contact with each other and they are instantly and permanently stuck together. You don't get second

chances to align things when using contact cement. I don't use contact cement at all.



Use for:



Laminating broad contact areas (sheeting on foam wings, fuselage doublers, etc.).



Don't use for:



Anything else.

Spray adhesive:



Fuel-proof: No.



Examples:



 3M 77

 Elmer's spray adhesive



Basically aerosol contact cement. Very convenient around the shop.



I use spray glue for a lot of tasks in the shop, but not on my models. I've noticed that it hardens and

crumbles after a year or so. I don't want my models falling apart in the sky. Some people say you can use

spray glue to attach sheeting to a foam core. Conversely, I've heard several stories about the sheeting

delaminating after a while.



Generally I use spray glue for attaching sandpaper to sanding blocks, patterns to wood, etc. When

attaching patterns, I spray only the pattern with a light coat and let it dry for several minutes. Then I attach

the pattern to the wood. The pattern will peel off easily after the part is cut out. When attaching sandpaper,

I spray the paper and the block and attach the paper when it gets tacky. I have to remove it with a heat gun

and then use solvent to clean up the glue. Spray glue is expensive, so I use try not to use any more than

necessary.



I recently priced these glues in a Home Depot store. The 3M 77 cost approximately four times more than

the Elmer's for the same quantity. Because I only use it for gluing sandpaper to sanding blocks, templates

to wood and similar non-critical bonds, I stick to the cheaper glue.



Use for:



 Attaching paper patterns to wood by spraying the template

only and allowing to dry just beyond a tacky state. Apply the

pattern to the wood, cut the part and peel off the pattern.

 Attaching sandpaper to sanding blocks by spraying the back of

the paper and the block and attaching the sandpaper when both

are tacky. Generally the sandpaper must be removed with a

solvent such as lacquer thinner. I use a heat gun to remove the

paper and thinner to remove remaining glue.



Don't use for:



 Any part of model construction - the stuff crumbles after about

a year.









Silicone adhesives:



Fuel-proof: Yes.

Flexible, relatively strong and fuel proof. It comes in quantities that no modeler will ever use and most of

it ends up being thrown out when it goes bad. The glue cures in the tube and is hard to remove to get at

good glue. The high-temperature variety can be used to seal mufflers and other engine parts (with care).



Use for:



 Sealing holes that you may need to access later. For example,

if you don't want exhaust residue to climb up the landing gear

leg and go into the wing, you can seal the landing gear area

with silicone.

 Attaching landing gear fairings so they flex instead of break

when the landing gear flexes.

 Wing saddles.

 Attaching servo trays inside the fuselage. I've heard from

others that it has been reliable for them although I've never

used it for this purpose. I don't trust silicone that much and

use epoxy instead.



Don't use for:



 General construction.

 Load-bearing members.









Glues for Plastic:



Fuel-proof: Yes.



Examples:



 Testor's plastic cement

 Tenax

 MEK (Methyl Ethyl Ketone)

 Model Master cement



Unless you've built a lot of plastic models, you probably aren't aware of the choices available for gluing

plastic. Most of us are aware of Testor's tube glue and that's about it.



All plastic glues pretty much work on the same principle. The glue contains a solvent (or is a solvent) that

melts the plastic together. In essence, the parts are welded together. This is why more glue isn't better.

The glue doesn't hold the parts together - it melts them together.



Generally speaking, I like medium viscosity glues such as Model Master cement. When using this type of

glue or tube glues, the glue is applied to one of the parts and then the parts are joined together.



However, when joining parts like cowls and wheel pants, I like the thin glues that can run along the joint

after the parts are joined. There are several glues of this variety as well. I use Tenax.









Thread Locking Compounds:

Fuel-proof: Yes.



Examples:



 Loctite (various formulas)



Unless you have a reason not to use it, always use the type that is called "Removable." I think it is the

"Red" formula, but I could be wrong about that. In any case, don't get the permanent kind - especially for

set-screws or you'll never get them out again. If you can put the part in an oven or have a torch then you

may be able to heat the part enough to get it loose, but if it's inside your R/C car, I doubt you'll want to put

your car in a 300º oven.



Use for:



Any metal hardware that you don't want to come loose.



Don't use for:



Plastic hardware such as nylon fittings.