Method for protecting printed data

A heat-transfer machine comprising:

• a feed system for feeding the machine with a plastic card,
• a feed roller on which a transfer film is wound, carrying at least one area of varnish to be deposited,
• a recovery roller on which the transfer film is wound after the varnish of the varnish area has been applied to a plastic card,
• a thermal head between the feed roller and the recovery roller,
• an ejection system for ejecting the plastic card from the machine after varnishing,
• a processor arranged to:
• control the simultaneous advance of the varnish area of the transfer film and of the plastic card under the thermal head and the heating of the thermal head in order to deposit the varnish,
• control the rewinding of the transfer film in order to position the varnish area upstream of the thermal head and the return of the plastic card upstream of the thermal head,
• control the simultaneous advance of the varnish area and of the plastic card under the thermal head and the heating of the thermal head at the level of the text and at the deformation in accordance with a sign scheme for making the deformation illegible.