Coating method and hardener for polyurethane paint

A coating method comprising applying a base coat paint comprising an active hydrogen-containing resin and a blocked isocyanate, without hardening the base coat paint by cross-linking, subsequently coating a clear coat paint comprising a hardener and an active hydrogen-containing resin on the base coat paint, hardening the base coat paint and the clear coat paint simultaneously by cross-linking, wherein the clear coat paint is a two-pack polyurethane paint with a mole ratio of 0.9 to 2.0 in NCO/H (active hydrogen), wherein the hardener comprises a polyisocyanate derived from 1,6-hexamethylene diisocyanate, satisfying all the following conditions:

• 1) the hardener includes an amount of diisocyanate monomers included in polyisocyanate of 0.25 weight % or less
• 2) viscosity of hardener when it is not diluted with any organic solvent is not more than 1000 mPa·s/23° C.,
• 3) content of 1,6-hexamethylene diisocyanate trimer in hardener is not less than 60 weight %, wherein the 1,6-hexamethylene diisocyanate trimer comprises an isocyanurate structure of HDI, an iminooxadiazinedione structure of HDI, or mixtures thereof, and
• 4) content of 1,6-hexamethylene diisocyanate dimer in hardener is less than 10 weight %.