Coating method for car body

The present invention provides a coating method for a car body in which a color base coating material, a pearl base coating material and a clear coating material are applied on the car body by a 3 coat 1 bake system making use of a 2 coat 1 bake coating line in which applied are a base coating material in the first stage and a clear coating material in the second stage and then two layer coating films thus formed are heated and cured in the same time, wherein the color base coating material and the pearl base coating material are applied in the first stage, and then the clear coating material is applied in the second stage; and subsequently, three layer coating films thus formed are heated and cured at the same time.

EXAMPLE 1 A color base coating material, a pearl base coating material and a clear coating material each described below were applied on a car body (made of a steel plate) on which a cationically electrodepositable coating material and an intermediate coating material were applied by a 3 coat 1 bake system using a 2 coat 1 bake coating line and heated and cured. Color base coating material: white color organic solvent type coating material comprising a hydroxyl group-containing polyester resin, a melamine resin, a titan white pigment and aluminum flakes. Used as the organic solvent was a toluene/xylene/methanol/ ethyl acetate mixed solvent having a boiling point of about 80 to 110° C. A Munsell color chart N value of the coating film: 8.0; a solid content: 35% by weight; and a viscosity: 12 seconds/Ford cup &num;4/20° C. Pearl base coating material: photo-coherent organic solvent type coating material comprising a hydroxyl group-containing polyester resin, a melamine resin and titanium oxide-covered mica flakes. A solid content: 35% by weight, and a viscosity: 12 seconds/Ford cup &num;4/20° C. Used as the organic solvent was a toluene/xylene/methanol/ethyl acetate mixed solvent having a boiling point of about 80 to 110° C. Clear coating material: organic solvent type coating material comprising a hydroxyl group-containing acryl resin and a melamine resin. A solid content: 35% by weight, and a viscosity: 20 seconds/Ford cup &num;4/20° C. The operation conditions of the reciprocating type automatic coating apparatuses for the horizontal parts and the vertical parts for applying the color base coating material and the pearl base coating material in the first stage were a conveyer speed of a coated article: 1.5 m/second, a reciprocating width of 1.0 m, a reciprocating speed of 51 m/minute, an air amount of an REA electrostatic coating gun (atomization/pattern): 2.5/3.0 kg/cm 3 , a distance from a head of the coating apparatus to a coated surface: 250 mm, a voltage of −60 KV and a coating material discharging amount of 300 ml/second. The operation conditions of the reciprocating type automatic coating apparatuses for the horizontal parts and the vertical parts for applying the clear coating material in the second stage were a conveyer speed of a coated article: 1.5 m/second, a reciprocating width of 1.0 m, a reciprocating speed of 51 m/minute, a revolution of an electrostatic coating bell: 25,000 revolutions/minute, a distance from a head of the coating apparatus to a coated surface: 250 mm, a voltage of −60 KV and a coating material-discharging amount of 200 ml/second. The color base coating material and the pearl base coating material were applied on the horizontal parts and the vertical parts of a car body on the conditions described above by means of the reciprocating type automatic coating apparatuses in the first stage of a coating line, and then the clear coating material was applied thereon on the conditions described above in the second stage. Thereafter, they were carried in a heating furnace and baked at 140° C. for 30 minutes to cure the resulting three layer coating films at the same time. The plural layer coating film thus formed showed the same or higher performances in terms of smoothness, clarity and a decorative design (color feeling, pearl feeling) as compared with those of films coated by a 3 coat 1 bake system in a 3 coat coating line and those of films coated by a 3 coat 2 bake system in a 2 coat coating line.