Ink jet printing device for applying a varnish forming a coating on a surface of a substrate, the device having a plurality of work stations and comprising at least: - an infeed magazine, - a discharge magazine, - a data-processing means for managing the operations on each of the work stations, - a means for moving the substrate between the different work stations, - a means for gripping and transferring the substrate from the infeed magazine to the means for moving and from the means for moving to the discharge magazine, - a plurality of nozzles housed in a module forming a printing head, at least one of the nozzles being supplied by a reservoir containing varnish to be ejected onto the substrate, each of the nozzles being made to vibrate by a piezo-electric actuator controlled by a device for controlling and regulating the form of the voltaic wave expelling the drop of varnish from the nozzle as a function of the viscosity and/or the composition of the varnish to be applied, such that the duration and power of excitation of the actuator determines the size and the shape of the drop of varnish, - an applying station for applying the product in a predetermined zone of the substrate, characterised in that - the reservoir contains varnish to be sprayed onto the substrate, - in at least one nozzle of the device incorporates a resistance heater connected to a control device for controlling the viscosity of the drop of varnish within the nozzle, said control device incorporating command and regulation means for the temperature of the drop of varnish within the nozzle, and - the device comprises a probe measuring the viscosity and/or at least one sensor measuring the temperature of the varnish downstream of the reservoir and upstream of said resistance heater. - an infeed magazine, - a discharge magazine, - a data-processing means for managing the operations on each of the work stations, - a means for moving the substrate between the different work stations, - a means for gripping and transferring the substrate from the infeed magazine to the means for moving and from the means for moving to the discharge magazine, - a plurality of nozzles housed in a module forming a printing head, at least one of the nozzles being supplied by a reservoir containing varnish to be ejected onto the substrate, each of the nozzles being made to vibrate by a piezo-electric actuator controlled by a device for controlling and regulating the form of the voltaic wave expelling the drop of varnish from the nozzle as a function of the viscosity and/or the composition of the varnish to be applied, such that the duration and power of excitation of the actuator determines the size and the shape of the drop of varnish, - an applying station for applying the product in a predetermined zone of the substrate, characterised in that - the reservoir contains varnish to be sprayed onto the substrate, - in at least one nozzle of the device incorporates a resistance heater connected to a control device for controlling the viscosity of the drop of varnish within the nozzle, said control device incorporating command and regulation means for the temperature of the drop of varnish within the nozzle, and - the device comprises a probe measuring the viscosity and/or at least one sensor measuring the temperature of the varnish downstream of the reservoir and upstream of said resistance heater. - the reservoir contains varnish to be sprayed onto the substrate, - in at least one nozzle of the device incorporates a resistance heater connected to a control device for controlling the viscosity of the drop of varnish within the nozzle, said control device incorporating command and regulation means for the temperature of the drop of varnish within the nozzle, and - the device comprises a probe measuring the viscosity and/or at least one sensor measuring the temperature of the varnish downstream of the reservoir and upstream of said resistance heater.