Composite paint film

An outer panel of an automotive vehicle body, comprising:

• a steel sheet forming part of the automotive vehicle body;
• a first paint film (I) coated on said steel sheet and formed of an epoxy-based cationic electrodeposition paint composition and having a minimum melt viscosity not less than 0.2 during hardening, said minimum melt viscosity being determined according to a logarithmic decrement of viscoelasticity of said first paint film and by an oscillated pendulum type viscoelasticity measuring device, said epoxy-based cationic electrodeposition paint composition including
• an amine-added epoxy resin (1) of the type which may be hardened with a blocked isocyanate,
• a colloidal silica containing gelled particulate (2) which is formed by dispersing a mixture of an acrylic copolymer having a hydrolyzable alkoxysilane group and a cationic group and a cationic acidic colloidal silica in water to carry out a cross-linking reaction in particles of said colloidal silica, and
• an alkyltin ester compound (3) of a liquid aliphatic carboxylic acid;
• and
• a second paint film (II) coated on said first paint film, said second paint film having a hardening starting time ranging from 15 to 20 minutes, said hardening starting time being determined according to a logarithmic decrement of viscoelasticity of said second paint film and by said oscillated pendulum type viscoelasticity measuring device.

A method for preventing corrosion of steel panels which comprises coating at least one surface of said panels with a composite paint film comprising

• a first paint film (I) formed of an epoxy-based cationic electrodeposition paint composition and having a minimum melt viscosity not less than 0.2 during hardening, said minimum melt viscosity being determined according to a logarithmic decrement of viscoelasticity of said first paint film and by an oscillated pendulum type viscoelasticity measuring device, said epoxy-based cationic electrodeposition paint composition including
• an amine-added epoxy resin (1) of the type which may be hardened with a blocked isocyanate,
• a colloidal silica-containing gelled particulate (2) which is formed by dispersing a mixture of an acrylic copolymer having a hydrolyzable alkoxysilane group and a cationic group and a cationic acidic colloidal silica in water to carry out cross-lining reactions in particles of said colloidal silica, an
• an alkyltin ester compound (3) of a liquid aliphatic carboxylic acid;
• and
• a second paint film (II) coated on said first paint film, said second paint film having a hardening starting time ranging from 5 to 20 minutes, said hardening starting time being determined according to a logarithmic decrement of viscoelasticity of said second paint film and by said oscillated pendulum type viscoelasticity measuring device.