A security document includes a substrate, and an imaging thermal coating on at least a portion of the surface of the substrate for thermal printing on the security document. The imaging thermal coating is soluble in one or more solvents. A warning message is printed on the substrate in an ink which is substantially insoluble in the one or more solvents. An obscuring coating on the substrate covers the warning message such that the warning message is not apparent to an observer. The obscuring coating is soluble in the one or more solvents, such that the obscuring coating is removed from the substrate if the document is subjected to washing with the one or more solvents to remove printed images on the imaging thermal coating. By this arrangement, an attempt to alter the security document by washing the security document with the one or more solvents is made apparent.