In place of mostly the traditional mild steel, sophisticated high strength steel sheeting and tailored structural pressed parts that compose the body are protected from corrosion by galvanising and other zinc coatings of varying types, according to the degree of protection required. In some cars, like the BMW 5-Series, mixed metals in the form of special steels and lightweight alloys are combined in the bodyshell, with careful sealing using special adhesives where otherwise they might make contact, to prevent any electrochemical corrosion resulting from their dissimilarity. In most finishing systems, five or more layers of coatings will be applied during the painting process. Before any painting though, the bare body shell is meticulously cleaned in solvent or detergent baths and dried to remove any contamination that might have been collected during manufacture – most critically any oils and greases that might compromise the adhesion of the paint subsequently applied. Following this, a solution of phosphoric acid and phosphate salts is applied, adding further to corrosion protection and improving the adhesion of the following coatings, by full immersion of the body in a tank of the phosphate.
After drying, a layer of tough primer, usually an epoxy-based paint, fills in any tiny scratches and small imperfections. This is almost universally applied in full immersion dip tanks where “electro-deposition” using electrostatic charging applies thicker paint layers to vulnerable areas, like sharp corners. Following this, operators, or now more frequently robots, apply plastic seam sealant that protects crevices and cavities at panel joins and welded joints from any moisture penetration that might initiate corrosion. This may be then followed by application in specific areas of sprayed sound-insulating material and underbody protective sealing. After application of further primer, decorative coloured coatings will then be applied, usually by teams of robots, and heat-dried to achieve a high-gloss finish. Finally the critical hard protective coat of clear lacquer is applied and dried by robotic spraying, with paint application again assisted by electrostatic charging of the body to optimise paint distribution. Clear coat not only protects the paint from the ultraviolet rays of the sun, it also acts as protection against small dings and scratches, with a small scratch in the clear coat much less visible than a scratch in the paint. Nissan has now developed a “self-healing” clear coat, using a partially elastic resin, where small scratches actually fix themselves and disappear in a matter of days. All these finishing steps are subject to some variation, and some companies have succeeded in combining two primer coats.
Many of the layers applied in previous times employed solvent-based paints are now using water-based coatings, which necessarily require heat drying and, where mixed water/solvent-based paints are used, a solvent collection and recovery system will be used. The painted body shells finally pass through illuminated inspection stations where experienced paint specialists and automatic measurement systems examine the body surface for its colour, gloss, and the depth of layer. The result of today’s manufacturing is a body with far greater protection from the elements, reduced labour input and use of toxic solvents, and more consistent application of the various coatings, where robots are used.