_ Primers may be applied by brushing, wiping or dipping. (Spraying may sometimes produce erratic results.) A thin uniform primer coating of approximately 0.01 to 0.02 mm (0.5mil) thickness usually provides the strongest bond. Care should be taken with plastic substrates such as polystyrene or polycarbonates (LEXAN®) which may tend to craze or become sticky when primer is applied. Crazing can be minimized if the primer is applied with a single, continuous stroke.
_ For most primers, a drying time of at least 30 minutes at room temperature is suggested prior to application of the RTV silicone rubber adhesive sealant. On porous surfaces, a second coat of primer may be required. Allow at least 30 minutes drying time between coats.
_ These primers improve surface wettability, inhibit corrosion, enhance bond strength and offer surface protection prior to bonding. They are often used to replace surface treatments and are capable of penetrating rough surfaces enabling effective sealing characteristics. Careful quality control procedures need to be strictly followed to ensure proper processing. Thickness of the primer is of vital importance. Additionally selecting the proper primer requires careful consultation with the adhesive formulator to ensure suitability with the adhesive/substrate combination and optimizing durability in service.
_ These primers are single component liquid systems and require no mixing. They can be easily applied by brushing, spraying or dipping. These low-solids, air drying coatings add a new thin layer to the surfaces being joined. At a later time period the adhesive can be applied. This has proven beneficial in numerous assembly applications. Primers often contain wetting agents, tougheners, corrosion inhibitors to improve serviceability. In fact elastomeric primers were developed to address the need for a flexible interface due to the thermal expansion issues. Most important, these products contribute to higher initial strength properties, long term strength retention characteristics and resistance to moisture/humidity attack.