ight, based on the resin content thereof, of an acrylic resin which is an acrylic resin containing at least 50 molt of recurring unit of the following formula (I-a), wherein R1 is an alkyl group having 1 to 4 carbon atoms, the thermally cured coating layer (II) being made of an organosiloxane resin formed from the following components a, b and c, (A) colloidal silica (component a), (B) a hydrolysi