English to Italian Tech/Engineering Construction / Civil Engineering paper based

An operation made in assembling panels of wood or other materials:
"THE MATERIAL is pressed and is not cast or extruded, so the thickness may vary from sheet to sheet or within a sheet. The surface on either side of a seam must always be indexed from the top and the difference and sanded on the underside. The aggressive sanding sometimes needed will expose several layers of paper, causing white lines to appear, which look like contour lines on a map. "