Some simple general rules are to be applied for successful bonding and an optimal use of adhesives and sealants.
Adhesives and sealants are made to stick!
In practice, adhesives and sealants adhere to almost all materials as long as they are mutually compatible. Therefore the choice of adhesive or sealant according to the materials to be joined is paramount. There are glues for virtually all known materials.
All adhesives and sealants do not have the same properties
Some will resist moisture or even immersion in water, others will be flexible or hard, some will be transparent or opaque or colored, others will withstand high temperatures or even fire for certain duration. Others will be suitable for food contact... Therefore, a glue or a sealant is chosen according to the envisaged uses (see: Science - Types of adhesives and their uses).
Plenty of glue or sealant does not mean better or stronger bonding
We must use the necessary amount, no more and not less. Some bonding will require thin layers (eg for materials with smooth or even surfaces) or thicker layers (eg for joining materials with irregular surfaces, or for voids to be filled using sealants). Putties in general and certain types of adhesives such as epoxy or polyurethane adhesives have the capacity to fill gaps or to join irregular surfaces satisfactorily.
Leave time to time. No glue, no sealant reaches its optimum resistance immediately
The fastest stick nearly instantly or in seconds, the slowest in several days. It is best to wait at least 24 hours for optimal adhesion. Some adhesives or sealants require maintaining the assembly with, for example, a clamped joint, as long as the adhesive or the putty sets and reaches a sufficient strengh.
Adhesive failure is almost never the result of glue or sealant, but a lack of preparation of the surfaces to be coated or an inappropriate choice of the product
Clean surfaces are a good guarantee of success. If the bonding breaks, with a simple visual inspection it is possible to determine the reason for failure and to remedy it (see: Science - Adhesion Failure).
Adhesives and sealants can be peeled off
To date, there are several techniques and products that make it possible to remove or clean adhesives and sealants. The most common methods are either mechanical removal (rubbing or abrading) or by using certain products which can soften or even destroy the adhesives and sealants and thus allow them to be removed quite easily with a spatula. Some glues, mastics or self-adhesives are suitable for temporary bonding and can be easily detached. (Let us note in passing that snails are a good example of temporary adhesion).
Tools and accessories
Some tools and accessories such as notched spatulas can also prove very useful for a better application and distribution of the adhesive or sealant.
For a more detailed explanation of how adhesives and sealants work, see "Science - Understanding Adhesion: How and Why".
Known for millennia (see Science - History of adhesives, adhesives and sealants), adhesives and sealants have become indispensable to fix, assemble or seal, in many areas, for new projects or repairs.