Surface preparation encompasses two main types of cleaning: a) detergent, b) solvent. The choice of cleaning method used will depend upon the type of substrate (metallic or nonmetallic) and the extent and nature of contamination. In some instances, it may suffice to merely dust or blow away loose dirt. In other cases, it may be necessary to remove all foreign material from the adherend surface.
Detergent, soaps, and caustic soda are the least expensive and easiest cleaning agents to handle. Applied by spraying, scrubbing, or immersing the part in an ultrasonically agitated solution, these cleaners can remove certain kinds of dirt and oil reasonably well. However, such cleaning agents may react with certain metals. After detergent cleaning it is necessary to rinse thoroughly with water and dry.
Figure 4 summarizes the ability of different solvent types at removing the most common surface contaminants.
Figure 4. Cleaning and degreasing with solvents
Solvent cleaning can be accomplished by wiping with a solvent-moistened cloth (or lint-free tissue), immersion in the solvent, or by exposure to the solvent vapor.
A ketone, such as methyl ethyl ketone, is generally a good solvent for cleaning metals, but it can be too aggressive for many plastics. An alcohol, such as isopropyl alcohol, is a better choice of solvent for use with plastic substrates.
Solvent cleaning should precede any chemical or abrasive surface pretreatment. Abrading a surface coated with oil, grease, or a release agent will serve only to drive the contaminants further into the substrate making it even more adhesion resistant. Further, grease and oil prevent the acid etch solution from reaching the adherend surface.