Primer

Primers are liquids that may be applied to a substrate prior to application of an adhesive or a sealant. The reasons for their use are varied and may include, either singularly or in combination, the following:

Protection of surfaces after treatment.Promoting chemical reaction between adhesive and adherend. Inhibiting corrosion of the substrate during service. Serving as an intermediate layer to enhance the physical properties of the joint and improve bond strength.
Being lower in viscosity than the adhesive or sealant, primers can be used to penetrate porous or rough surfaces to provide better mechanical interlocking and for sealing such surfaces from the environment. Primers are often applied and appear as protective surface coatings. Application processes and equipment for applying primers are similar to those used in applying a paint coating to a substrate. The application of a primer is an additional step in the bonding process, and it comes with associated costs and quality control requirements. Therefore, primers should only be used when justified. The most likely occasions when a primer is needed are: when the adhesive or sealant cannot be applied immediately after surface preparation, when the substrate surface is weak or porous, or when the adhesiveadherend interface requires additional protection from environments such as moisture.