Two dimensional reflection intensity distributions of terahertz wave intensity provide a direct transparent image of corrosion beneath opaque anticorrosion paint layers. However, attention needs to be paid to changes in effective reflectivity due to uneven surfaces. This study demonstrates in-situ estimation of surface roughness and reflective index by frequency analysis of the terahertz wave form reflection echoes. Deconvolution technique is applied to resolve the undulation of the waveform to separate the overlapping of the reflections from the front and back surfaces of the paint. Estimated and measured roughness values are in good agreement, to show roughness and refractive index distribution of a service-used transmission tower cross-arm.