Here are some useful tips and techniques to consider when working with glues. It's not an exhaustive list covering every type of surface and glue, but hopefully it may help or inspire creative problem solving for your own gluing challenges.
Read the label on your particular glue or adhesive to see if it has any specific instructions regarding surface preparation. Otherwise, you can refer to this list of common techniques to prepare surfaces for gluing.
Some glues work better with smooth surfaces. This tends to be glues that are absorbed into porous surfaces -- the absorption gives the glue plenty of material to grip, so sanding the the surfaces means the surfaces fit together better and the glue line layer can be thinner. A thinner glue line layer for PVA wood glue, for example, is much stronger than a large, thick glue line layer. So for glues like this, it's best to sand the joint surfaces with at least 200-grit sandpaper.
Other adhesives need a rough surface to adhere properly. This tends to be adhesive/material combinations where the adhesive does not get absorbed by the material. Roughing the surface provides more surface area for the adhesive to attach. A good example of this is 2-part epoxy and metal, glass, or plastic material. These materials are non-absorbing, so it's best to roughen those surfaces with 80 to 100-grit sandpaper to provide the best adhesion.
As mentioned above, some materials are supposed to be able to absorb an adhesive, like wood and PVA wood glue. But in some cases, a wood surface can become non-absorbing, like when wood is burnished (rubbed to a glossy surface with wood shavings or oil) or burned. But you can test to see if wood will absorb wood glue by just applying a few drops of water to the wood surface and see if they bead up or are absorbed by the wood. That will provide a good indication of whether a glue will be absorbed properly or not.
Some materials and glues are highly sensitive to the moisture content of the material. Usually it's best for the surfaces in a glue joint to have the same moisture content. It's hard to exactly measure moisture content of a material without a moisture meter, but in general it's good to let materials acclimate in an environment with known humidity for a few days for best gluing results. Some glues work best with drier or wetter materials. Polyurethane glue, for example, usually requires a moisture content of >10% when used on wood. Low humidity conditions (e.g. during wintertime) could be responsible for a polyurethane wood glue failure.
Various chemicals and oils can inhibit glue adhesion. It's best to clean gluing surfaces as recommended by a particular glue. Sometimes this is with a damp cloth, by sanding, using steel wool, or using emery cloth. Cutting wood can release sap and oils, so one must wait for this to equalize and clean up the cuts before gluing.
Different glues may be more or less tolerant to gaps, and engineers describe some glues specifically as "gap filling" glues. A plastic solvent, for example, "welding" together acrylic depends on the ability to melt the two acrylic surfaces together, which works best when there are no large gaps to bridge. For situations like plastic solvents, the best solution is to manually sand or cut joint pieces so they fit together tightly.
In other cases, a glue can actually contribute to filling gaps in a joint. For example, CA (cyanoacrolate, a.k.a. Super or Krazy glue) glues can be combined with baking powder to create an accelerated, cured compound that can help to fill in gaps in a joint.
It can be hard to gauge the exact amount of glue necessary for a joint. When clamping a joint, for example a wood joint with PVA wood glue, some glue can be squeezed out of the joint. One way to make this undesirable glue squeeze-out more manageable is to use something like masking tape or painter's tape to cover the surface along the joint where squeeze-out is expected. Because the glue will stick to tape instead of the surface, it's much easier to remove the squeeze-out.
Adhesives aren't finger paints. More than likely you'll want to use the bottle/tube the glue came in or some other tool to apply the glue directly to the surfaces of the joint. Here are a few options with suggestions on how to use them.
Use this to spread glues like PVA glues or polyurethane glues. Try to wash the foam brush as much as possible afterwards with water or an appropriate solvent. If the foam becomes damaged, you can remove the foam and still use the handle and plastic core, as described below.
This is also good for spreading glues like PVA glues or polyurethane glues to get a thin, even layer of glue. It's a good way to get extra use out of foam brushes.
Fine tip syringes are commonly used for very thin glues, like thin CA (cyanoacrylate) glues or sometimes plastic cements. These have to be cleaned well, or else you'll have to try to unclog them the next use.
These are cheap, disposable ways of applying CA (cyanoacrylate) glues, epoxies, PVA glues, or polyurethane glues.
An index card or a piece of card stock can be a cheap, disposable glue applicator or spreader.
Hot glue guns are necessary when working with hot glue sticks. These are electrical appliances, so care must be taken when working with any highly flammable materials (unplugging the glue gun to avoid sparks). But you get great glue stick shelf life, instant application time, quick cure time, etc. You just have to avoid burning yourself...
Some glues can be thinned, either to change the glue properties (like increasing working time) or to make use of the glue more cost effective. For example, one popular glue is "Mod Podge", which can be approximated by mixing a 50/50 ratio of water and PVA white glue. Or CA (cyanoacrylate) glue is sometimes thinned with acetone by model plane builders and others.
Judging the right amount of glue to use in a joint is more art than science. But one key thing to keep in mind is whether the glue expands or contracts as it cures. For example, polyurethane glue (commonly used for wood joints) can expand up to 3x its original size. PVA glues will shrink slightly while curing. So in general, you'll use much less polyurethane glue on a joint than PVA glue.
Applying glue to both sides of a joint is a good way to ensure coverage of the surfaces. However, one consideration is the working time of the glue. Applying glue to only one side of a material will take half the time (one side vs. two). So if the glue you're using is going to set very quickly, it can be preferable to only apply glue to one side and rely on clamping pressure or slight rubbing of the two surfaces to spread glue throughout the joint.
With some glues, it's advisable to wait a short period time after applying the glue for it to become partially cured and tacky. A glue in its tacky state can make it easier to accurately position the second surface to form a joint, and the tackiness can help hold the joint in place. One must be careful when using this approach to not wait too long; if the glue cures too much before bringing the surfaces together, the glue joint may not hold very strongly.
Some glues are temperature dependent, in that their cure times can vary based on the ambient temperature. When applying an adhesive, it's best to verify that the ambient temperature is within the glue's allowed temperature range. If the temperature is near the extremes of the allowed range, the working time of the glue may be affected, meaning that the glue joint must either be made very quickly, or conversely, the glue joint may need to be clamped longer than normal to fully set.
Some glues are toxic. For most adhesives, it's best to work in a well-ventilated area. Wearing safety goggles is always advisable. A breathing mask and chemical filter could be used for very toxic glues. And chemical-resistant gloves (like nitrile gloves) can be worn to avoid skin contact with an adhesive.
That being said, some glues are safer than others. Read the label or acquire the MSDS for the particular glue to ensure you're taking all appropriate safety precautions.
Clamping is usually necessary for glues with a long working time, if just to avoid having to manually hold a joint in place. In addition, some glues are stronger when the cured glue line layer is thinner, and clamping a glue joint can help make the glue line layer thinner. Clamping is probably not necessary for any quick-setting adhesives, especially for ones that would set faster than a clamp could be applied and tightened. To avoid clamping, some adhesives can be combined with an accelerant to make them set faster, which means manually holding the joint together may be feasible.
The amount of pressure to use when clamping varies based on the material and glue used. Some glue manufactures will provide specific recommendations for the pressure (in psi) that should be applied while a glue is curing for particular materials. Usually more pressure will result in higher bond strength. Some glues, however, can "starved" when too much pressure is applied to a glue joint. For example, when gluing wood with epoxy, it's usually preferable to use a light pressure to avoid squeezing out too much epoxy from the joint.
Precisely measuring pressure is usually done in more industrial applications with hydraulic or pneumatic pressure applied. Any true load-bearing engineering application should be carefully calculated and executed to make sure glue joints will be strong enough given the pressure at cure time.
For more casual gluing, you basically want to check whether your glue/material combination is one that benefits from more pressure or whether you should avoid heavy pressure. So for wood and epoxy, you'd want light pressure. For wood and PVA glue, you'd want to apply as many hand-tightened clamps as necessary to bring the joint tightly together.
One tip for applying light clamping pressure is to use thick soft rubber pads between the clamps and material. That will help spread the clamping pressure and ensure gentle, light pressure.
A glue joint usually should remained clamped for the entire cure time. Keep in mind that a particular joint may cure faster or slower depending on its geometry. A glue that cures via exposure to air may take longer to cure in a very large glue joint because it's harder for air to reach the center of a large joint.
Use a piece of scrap material between the clamp and your piece to avoid clamp marks, either from dirty clamps or the clamp pressing into the piece too much. Larger scrap material will spread the pressure out more. Thick soft rubber pads can be used to provide light clamping pressure that won't leave marks on the piece.
Scrap material will also prevent glue squeeze-out from getting onto the clamps. Masking tape can also be used on the clamps to keep them clean and dry.
Sometimes a joint may not be easy to clamp depending on its particular geometry. In such cases, one may need to actually build a jig to hold the piece in a position suitable for clamping or a jig that will allow the transfer of clamping force to a peculiarly shaped object.
One interesting technique that accomplishes nearly the same goal as clamping is using another, quick-setting glue to hold a glue joint together. For example, you could use a couple spots of quick-setting CA (cyanoacrylate, a.k.a. Super or Krazy) glue in addition to a larger area of regular PVA wood glue to glue pieces of wood together without clamps. The CA glue spots could set while the pieces are held together manually, and those spots could help "clamp" the piece together while the PVA wood glue cures.
"Thermosetting" adhesives will cure faster with additional heat. Sometimes this will just be turning up a heater in a room, laying an electric blanket over the joint, leaving the joint in the sun, or using an oven.
Some glues can be combined with a hardener substance that causes the glue's normal cure time to happen much faster by speeding up the normal chemical reaction. Sometimes these are commercially available, but in other cases they can just involved household substances.
Baking soda, for instance, when combined with a CA (Cyanoacrylate) glue (a.k.a. "Super" or "Krazy" glue) will harden very quickly. A sprinkle of baking soda over a CA glue joint can speed up the curing, or baking soda can be used to created a hard filler in a crack in a joint when the glue is applied. This chemical reaction creates a lot of heat, so be careful not to burn yourself.
Some glues cure with exposure to air. Using a ventilation fan can help these glues dry faster. Sometimes a low-moderate heat air source (hair dryer) can provide a stream of low-moisture air that helps a particular glue cure faster.
Some glue squeeze-out is workable after it cures, in the sense that it can be mechanically modified. For example, polyurethane glue squeeze-out can be sanded into a powder and wiped away. A 2-part epoxy can also be sanded, filed, or ground (using a grinder) to help clean it up.
Some glues aren't waterproof. While this can be a problem if the glue is exposed to the environment, it does make cleanup easier. For example, normal white PVA glue can usually be cleaned up with just warm, soapy water.
CA (cyanoacrylate) glues dry fast, hard, and clear. They can work well as a protective glossy finish. Wood turners -- people who use a rapidly rotating wood lathe to make objects -- sometimes use CA glue as a finish because it will stick to a turning piece on the lathe and spread out evenly.
It's important to know what reaction causes your glue to cure. It could be air, heat, UV light, or even the absence of air/oxygen (for example, Loctite adhesives). If you know what causes it to cure, you can try to store the glue in a place that has opposite properties. A cool, dark cabinet or shelf is a typical suggested storing location.
Even if the glue doesn't cure in the bottle, it may cure at the bottle's tip or on the applicator. Sometimes it's best to wipe off the bottle/applicator after use to minimize problems. It's also possible to use a pin or an awl to poke a hole in a bottle that's sealed itself. Sometimes a thinning or cleaning agent (like acetone for some glues) can be used to help avoid glue hardening on the bottle or applicator.
Some users recommend the refrigerator as an ideal location to store glues like CA (cyanoacrylate) glues. For other glues, the refrigerator may be too cool of a location or not have enough fresh air (for anaerobic-curing adhesives like Loctite). The main danger with using a refrigerator for CA glue storage is that fumes may be released that could contaminate food stored in the same refrigerator, or possibly someone could mistakenly use the CA glue for the wrong purpose. Unless glue storage is a huge problem for you, a cool, dark shelf or cabinet is probably better than your home refrigerator for glue storage.