PURPOSE: To raise the temperature efficiently by hot water in the range not to cure the varnish but to improve penetration of the varnish, by providing a substrate applied or impregnated with the varnish with hot water and then drying it.

CONSTITUTION: A hot water bath 7 as a hot water providing device 5 which stores hot water 6 is arranged behind a varnish bath 1 and a dryer 4 is installed behind it. Varnish 2 is made by melting thermosetting synthetic resin like phenolic resin and being added with solvent to lower the viscosity. A substrate 3 is a sheet material like the kraft paper. The substrate 3 is dipped in the varnish bath 1 by a guide roll 8, the excess of the varnish being removed by a roll 9, dipped in a hot water bath 7 and impregnated efficiently with the varnish, then the hot water and the solvent of the varnish are evaporated and removed in a dryer 4. Application of water may be done by means of showering etc. besides dipping.