The procedure is simple: take a surface plate or precision straight edge and a worktop that is relatively straight, then stick sandpaper on it to sand out the rougher unevenness, grease the straight edge with a mixture of machine oil + blue pigment, place it on the surface with a wallpapering roller and move it around to show the high spots and by grinding them and removing them with a chisel, we get relatively high-precision straight surfaces.
