Process for forming multi-layer coatings

Claim 1

A process for forming a multilayer coating including at least two layers on a metal substrate which consists of coating at least portions of said substrate with an undried lower coating layer of a powder coating composition which is at least one member selected from the group consisting of (A) a thermoplastic resin which is selected from the group consisting of vinyl chloride homopolymers and copolymers, polyethylene or polypropylene resins, nylon resins, linear saturated polyesters, cellulose derivatives and chlorinated polyethers, and (B) a thermosetting resin which is selected from the group consisting of an epoxy resin composition comprising (1) at least one epoxy resin selected from bisphenol type, novolak type and alicyclic type epoxy resins and (2) a curing agent selected from dicyandiamide or derivative thereof, acid anhydride, aromatic diamine, BF 3 -amine complex, imidazole and dihydrazide, a polyester resin composition comprising (a) a polyester which is prepared by a condensation reaction of polyhydric alcohol and at least one of a saturated polybasic acid and an unsaturated polybasic acid, and (b) a curing agent selected from acid anhydrides, melamine resins, blocked isocyanate compounds, a compound having at least two glycidyl groups in the molecule, and ethylenic crosslinking monomers, acrylic resins and curing agents, and self-crosslinking acrylic resins, and before melting said powder coating applying thereon at least once a slurry paint comprising thermosetting synthetic resin particles to form a laminate coating, the slurry paint comprising 10 to 70% by weight of resin particles dispersed in a medium composed mainly of water, said resin particles having an average particle size of 4 to 30μ and a softening point of 5° to 120° C., and baking the resulting laminate coating at a temperature higher than the film-forming temperature of the coating layer having the highest film-forming temperature among respective coating layers.