Adhesives and Fillers
The single biggest error most people make with adhesives is that they use too much!
Obviously, you must use enough of the proper adhesive to do the job, and you certainly don't want to compromise the integrity of your model, or anyone's safety by not using enough adhesive, but...
Modern glues, especially Cyanoacrylate ("CA") have been one of the most important advancements in the hobby that I can think of. I began building models out of balsa in the mid 60s, when we mostly used standard "wood glue", meaning the Ambroid type, or white glue, "carpenter's glue",etc.. This was a nightmare at times, because, although it's easy to sand when dry, it can warp wood, due to it's water content, takes a long time to dry, allowing parts to slip out of alignment, and generally slowed the modeling process to a crawl. Now we can "instantly" stick things together and go fly! Still, everything has it's good and bad points...Here are some tips.
Getting back to those "old fashioned" wood glues...
Don't disregard these. CA is NOT always better, and here are some reasons why:
First of all, we're all aware of the issue of nasty fumes with CA. Wood glues don't have that problem.
Let's say you're building a very light structure, using contest grade balsa, in a jig. Wood glues are slow enough to allow repositioning in the jig. They are also much more sandable than CA. If you make any errors with CA, chances are that you'll have a bump that will be impossible to remove without damaging some of the surrounding wood. Next, consider the engineering of the model. If you have a very light structure, and the idea is to spread the loads evenly throughout it, CA makes a much stronger "spike" or stress point that defeats that purpose. Another thing to consider is that CA is more brittle than wood glue, and so your structure may break prematurely, rather than flex a little, thus absorbing and distributing the loads. One final thing to consider, if it matters to you, is that because the wood glues generally dry clear and are sandable, your structure may look neater if you use wood glue. Many world-class modelers and manufacturers use wood glue instead of CA, and it's not because they're dumb!
Some basic engineering concepts
You'll see the word penetration used many times in the following discussion. This is a vital concept in composites. Here's an example which shows the strength of a typical composite, such as a balsa-sheeted foam wing core. First, look at the strength of the foam core. It can be easily broken with your fingers. but pick up a piece of foam with proper balsa sheeting and you'll find it enormously strong. The foam only provides shape, and a gluing surface. The balsa has enormous strength in tension, moderate strength in compression (with the grain) and so converts bending loads in a wing to tension and compression. What happens if the sheeting isn't properly glued to the foam, or separates over time, due to vibration, for example? You have only the foam once again taking the loads, and it will break, probably causing a crash, and definitely creating a safety hazard. That's why penetration is an important quality in adhesives. The glue must be able to grip both the foam and the balsa, and not separate later, due to repeated flying! (Same is true for fiberglass.)
Pick the right glue for the job.
CA is great for wood and other porous materials, especially if you're in a hurry. Basically, the thickness of the glue varies with it's curing time. The "instant" glues tend to be very thin, "medium" curing is a little thicker, and "slow" is thicker yet. The "thin" type is good when you need to penetrate the wood and get a quick bond. "Medium" is often referred to as "gap filling", since it clings to the surface a little more than the "thin" type, and can produce a slight exterior fillet, which can be a stronger joint. The "thick" type cures more slowly, penetrates the wood less, and gives you more time for final alignment of parts. All of these types can be accelerated with the aid of a liquid (usually a spray bottle) to make it cure almost instantly. Use the accelerator with caution, as this stops the penetration of the glue, and most people agree that it produces a weaker joint. Generally, there's no time (and no need) to use a filler with CA glues, although you might be able to do it with practice. Non-fiber filler, such as micro-balloons weakens the joint, and should not be used for important structural joints. This is true for all glue types.
Epoxy
2-part epoxy, also available in different cure rates, has different qualities. The faster "5-minute" epoxies generally are thicker, and weaker than the longer-setting, thinner-consistency epoxies. Remember that thinner glues penetrate the wood (or cloth) more, and so they generally make stronger bonds. Epoxy itself is not a strong structural material. It's strength comes from the materials it can penetrate, and that's a fundamental truth with composites.
The strength of a fiberglass structure is achieved by the (relatively weak) epoxy holding the (very strong) fabric fibers in position! Use only enough epoxy to do this job. Excess epoxy on the surface of fiberglass only adds weight and not strength. This holds true for all adhesives. Bond the parts, but do not allow excess glue to remain around the structure. Wipe it off before it cures, using a paper towel and alcohol, and you'll not only have a strong, lightweight structure, but a neat looking one as well.
ALWAYS prepare a fiberglass surface before using any adhesive on it, by first cleaning it with a little alcohol or acetone, followed by roughing it up a bit with sandpaper. It's the nature of molded fiberglass to have a wax-like residue and/or mold release agent on it, and you want to make sure that your adhesive will stick, and stay stuck!
A great adhesive for vibration resistance
Another useful adhesive, especially when attaching formers and servo mounts to fiberglass, is silicone. After preparing the fiberglass, apply a bead of silicone on both sides of the item, where it attaches to the fiberglass, and smooth it out into a fillet, using either your finger (wearing gloves!) or a rounded tool of some kind. (Popsicle sticks are great.) It takes at least 24 hours to cure, but is an excellent choice for a vibration-resistant mounting.
Have you ever had engine mounting screws or muffler screws come loose, due to vibration? Next time, simply put a drop of silicone on the tips of those screws before you screw them in. It's very easy to unscrew them when you need to, but they won't come loose on their own. I also apply a thin coat of silicone to the exhaust manifold, where the muffler attaches, to provide a complete seal, and eliminate metal-to-metal contact. I've been using this techniques for several years with no problems.
Fillers
Fillers fall into two broad categories, structural and non-structural. It's important to realize the difference.
When you want to create a cosmetic (non-structural) component such as a wing fillet, fillers can be useful. Epoxy is very difficult to sand, and the filler does two things to help you. First it makes the epoxy "thixotropic", which means simply that it's thicker, and will cling to surfaces without running, better than the epoxy alone. Also, when cured, it's a lot easier to sand, and lighter, as an added bonus.
The term "micro balloons" has been used to describe at least two types of filler that are very different. There are glass micro balloons. roughly the consistency of baby powder, but actually made of microscopic glass beads. These are white in color and can be mixed to produce a very hard, or a medium-hard surface, which is easier to sand than epoxy alone. Then there are phenolic micro balloons, made of (surprise!) phenolic powder. This is usually a reddish, clay color, and can be mixed to produce a surface which is easier to sand, albeit not quite as durable, as glass micro balloons. Both of these are non-structural.
When you need a very strong joint, and want to "mold" a fillet to encapsulate an item, such as a firewall or main fuselage former, you should use a filler that contains structural fibers, such as small pieces of milled fiberglass. This is sold under a variety of brand names, or you can make your own, from scraps of fiberglass, Kevlar, or carbon fiber cloth you may have. Chop it up into short pieces (1/8 inch to 1/4 inch, depending on the size of your structures) and mix it thoroughly with your epoxy. Use your finger (wearing gloves!) or a rounded tool to make a nice fillet on both sides of the firewall or former, before the resin cures, and you will have a very strong joint.
There are probably hundreds of variations on adhesives and fillers available today, and many manufacturers market these common types under their own "special" brand name. These are the basics.