A method and system for thermal printing of secure documents. A contact thermal printing system includes: a) thermal print head having at least one heater element; and b) a thermal printer controller. The printer controller is programmed to: 1) move the print head in a predetermined pattern while the print head is in contact with a top coating of a thermal print medium; 2) overdrive the heater element during predetermined parts of the pattern to a temperature sufficient to soften or melt the top coating, thereby forming a pattern of striations in the top coating; and 3) control the heater element to print at least part of an image on the medium. The contact surface of the print head can have a specific pattern of features for forming the striations, whereby a pattern of striations formed can encode information both in the direction of the motion of the print head and transversely to the direction of motion. The pattern of striations can form a bar code. The image can be a postal indicium printed on a document which is a mail piece, or other documents having value, such as tax stamps and tickets, can be printed.