is prepared by adding two doses of hydroxylated polyacrylic resin or hydroxylated polyester resin or hydroxylated polyether resin with a low molecular weight and a dose of aliphatic or aromatic tri functional polyisocyanate for example polyacrylic resin u201c ref u8560 u201d and trimerised hexamethylene diisocyanate u201c ref 8580 u201d sold by the company chimitec france the quantity of the mixtures spread on the silk screen for the second printing is adjusted to correspond to a thickness of between 10 u03bcm and 20 u03bcm the silk screen used is such that the second imprint is of the continuous type ie it forms a continuous layer 5 which covers completely the first discontinuous imprint 3 fig2 and thus constitutes a first layer of a film it will be appreciated that a single silk screen can be used to produce a plurality of films simultaneously each layer 5 of the second imprint has a peripheral dimension and shape corresponding to those of the portion of surface of the document to be protected the second imprint is produced such that the thickness of the layer 5 formed is preferably less than 20 u03bcm or approximately 20 u03bcm advantageously the first and second imprints are produced such that the thickness of the varnish 3 formed by the first imprint is between one third.