field may then be increased by decreasing risks of forming of electric arcs between the electrodes the impregnation of the porous substrate with the powder may thus be performed faster and or deeper an alternative configuration would combine a cylindrical electrode on which an insulating conveyor having the cavities on its external surface would be assembled of course the above described excitation devices may be completed or replaced with other devices favoring the displacement of the powder across the porous substrate fig3 shows an example of an excitation device of vibrating type comprising freely rotating bars 40 42 44 46 and 48 of polygonal cross section having their diameter selected according to the thickness of the porous substrate and to its running speed for example the bars may have a hexagonal cross section with a diameter in the range from 10 to 20 mm the bars are arranged parallel to cylindrical conveyor 12 and sufficiently close to its surface to allow their pivoting on passing of porous substrate 8 thereby the bars apply a variable pressure on the substrate generating vibrations in said substrate which favors the migration of the powder thereacross to adapt the pressure applied by the bars according to the porous substrate and to the depth of impregnation with the powder other embodiments are possible by modifying at least one of the following characteristics the distance between bars and the cylinder surface the shape of the bars their rotation speed etc other known excitation devices may be envisaged to favor the impregnation of the porous substrate with the powder such as for example devices of acoustic ultrasonic aeraulic pneumatic etc type fig4 shows another embodiment which differs from the previous examples by the replacing of cylindrical conveyor 12 with a belt conveyor 50 the.