Paint composition for pre-coated metal and method of curing treatment of pre-coated metal by using the composition

A method of treating a surface of a steel sheet, the method comprising the steps of:

• (a) forming a coating layer by coating the surface of the steel sheet with a paint composition comprising a binder resin selected from the group consisting of a polyester resin, an alkyd resin, an acrylic resin, and a modified resin thereof, wherein the binder has a hydroxyl value of 5 mgKOH/g to 100 mgKOH/g and a molecular weight of 1,000 to 25,000;
• (b) performing primary curing on the coating layer by heating within a temperature range of 180° C. to 300° C.;
• (c) molding the steel sheet having the primarily cured coating layer formed thereon by mechanical processing;
• and
• (d) performing secondary curing on the molded steel sheet by heating within a temperature range of 140° C. to 200° C. to recover physical properties of the coating layer damaged in the molding.