APPLYING PAINT COATINGS

16.5 APPLYING PAINT COATINGS

The prime coat should be applied to the dry metal surface as soon as possible after the metal is cleaned in order to achieve a good bond. Better still, the metal should fi rst be given a phosphate coat (see Section 15.3 ), in which case the prime coat, if necessary, can be delayed for a short while. The advantages of a phosphate coat are a better bond of paint to metal and good resistance to undercutting of the paint fi lm at scratches or other defects in the paint at which rust forms and progresses beneath the organic coating. For many years it has been standard practice to coat automobile bodies and electric appliances with phosphate before painting.

Only in unusual cases should paint be applied over a damp or wet surface because poor bonding of paint to steel results under these conditions. A second prime coat can be applied after the fi rst has dried, or a sequence of top coats can follow. A total of four coats with combined thickness of not less than about

0.13 mm (5 mils) is considered by some authorities to be the recommended minimum for steel that will be exposed to corrosive atmospheres [10] . During the past 30 years, there have been vast improvements in the corrosion performance of automobiles, driven not only by the demands of the public for increased corrosion resistance, but also by the Clean Air Act of 1970 [with regula- tions regarding volatile organic content (VOC)] and subsequent revisions. Tougher and thicker materials have been developed, and the application processes have been improved. One of the most important advances has been the development of the cathodic electrodeposition priming process, introduced in 1976 and, by 1985, fully implemented by most vehicle manufacturers in their assembly plants. Today, nearly 100% of the cars and trucks manufactured in the world are primed using the cathodic electrodeposition process [11] . Following phosphate pretreatment, the vehicle is immersed in the electrodeposition bath for typically 2 – 3 minutes

[11] . Electric current is applied, and a fi lm is deposited. One of the advantages of this process, compared to spraying, for example, is that it results in a uniform, thin coating [about 25 μ m (1 mil) thick] with coverage on both exterior and interior cavity surfaces. In addition, this process is control- lable, automated, effi cient, and environmentally acceptable [12] .

ORGANIC COATINGS

16.5.1 Wash Primer

A wash primer, WP1, was developed during World War II in order to facilitate the painting of aluminum. Subsequently, it was also found advantageous as a prime coat for steel and several other metals. One wash primer solution consists of approximately 9% polyvinyl butyral and 9% zinc tetroxychromate by weight in a mixture of isopropanol and butanol as one solution, which, just before using,

is mixed with a solution of 18 wt.% H 3 PO 4 in isopropanol and water in the weight ratio of four of the pigmented solution to one of the latter [13] . The mixture must

be used within 8 – 24 h after mixing. It has the advantage of providing in one operation, instead of two, a phosphating treatment of the metal and the applica- tion of the prime coat. It has proved to be an effective prime coat on steel, zinc (galvanized steel), and aluminum.

16.5.2 Painting of Aluminum and Zinc

Paints do not adhere well to aluminum without special surface treatment, unless the wash primer is used, which provides its own surface treatment. Otherwise, phosphating or anodizing is suitable. The prime coat should, in general, contain zinc chromate as an inhibiting pigment. Red lead is not recommended, because of the galvanic interaction of aluminum with metallic lead deposited by replace- ment of lead compounds. Paints pigmented with zinc dust plus ZnO (zinc - rich paints) can also be used satisfactorily as a prime coat, forming a good bond with the metal. In this case, Zn and ZnO apparently react beforehand with organic acids of the vehicle, ensuring that aluminum soaps and other compounds do not form at the paint – metal interface to weaken the attachment of paint to metal.

Zinc or galvanized surfaces are also diffi cult to paint and should be phos- phated beforehand, or a wash primer should be used as a prime coat. As with aluminum, zinc chromate, but not red lead, is a suitable inhibiting pigment in the prime coat, or zinc - rich paints can be used. A coating system consisting of painted galvanized steel provides a protective service life up to 1.5 times that predicted by adding the expected lifetimes of the paint and the galvanized coating in a severe atmosphere, and the synergistic improvement is greater for mild environ- ments [14] .