Multimode planar polymer waveguides were fabricated on hybrid polymer nanoporous layers with a fast industrial aqueous-based method well adapted to coat large-surface plastic substrates and easily scalable up to economical mass production volumes. Using a fiber probe internal light source technique, we were able to measure quantitatively the modal propagation losses. In particular, attenuation coefficients lower than 0.5 dB/cm were found for the fundamental modes, thus showing the potential of our polymer nanoporous layers as optical isolation layers in waveguiding applications.