Abstract: This article presents the assessment of four different 3D-printed rectangular waveguide structures using highly conductive filaments, operating from 8 GHz to 40 GHz. The waveguides are evaluated in terms of the high-frequency transmission losses and are fabricated by using a highly conductive filament with a fused filament 3D printer. A smoothing treatment based on chloroform is used on the waveguides in order to reduce the roughness introduced by the 3D printing process. The measurement results confirm that the high-frequency transmission losses are reduced with this posttreatment, having a higher impact in higher frequencies, in which a reduction of around a 65% with respect to the nominal value is achieved.