A method of preparing a security laminate including the steps of: a) providing a polyethylene terephthalate substrate (1); b) stretching the polyethylene terephthalate substrate in either a longitudinal or a transversal direction; c) coating and drying a first coating composition (2) on the stretched polyethylene terephthalate substrate; d) stretching the coated polyethylene terephthalate substrate in the longitudinal or transversal direction not selected in step b) in order to obtain a coated biaxially stretched polyethylene terephthalate substrate having a layer of the first coating composition with a dry thickness between 50 nm and 400 nm; e) coating and drying a second coating composition (3) on top of the dry layer on the biaxially stretched substrate, wherein the first coating composition contains a copolymer selected from the group consisting of a hydroxyl—functional, partially-hydrolyzed vinyl chloride/vinyl acetate copolymer and a polyester-urethane copolymer; and wherein the second coating composition contains a hydroxyl—functional, partially-hydrolyzed vinyl chloride/vinyl acetate copolymer. Security laminates and security documents containing them are also disclosed.