As a general rule, all substrates must be treated in some manner prior to bonding if not to remove or prevent the formation of weak boundary layers then to provide a consistent and repeatable surface. Surface preparation can range from simple solvent wiping to a combination of mechanical abrading, chemical cleaning, and acid etching.
This article will focus on cleaning as a passive method of surface preparation. Sometimes cleaning is the only surface preparation step required depending on the type and condition of the substrate and the requirements of the bond. When structural bonds are required, cleaning often is a process that comes before and then again after more aggressive processes that physically or chemically change the surface.
The main purpose of surface preparation is to ensure that adhesion develops to the extent that the weakest link in the joint is either in the adhesive or sealant or in the adherend. With optimum surface treatment, failure should not occur at the interface because of a weak boundary layer or insufficient wetting.