For terahertz sensors to enter the industrial sector, idealized measurements on simplified samples are not sufficient. We report on further development of terahertz systems for measuring the thicknesses of multiple paint films for non-contact quality control in automotive manufacture. Analysis of time-domain data is described, and the results of measurements made on real industrially-applied wet-on-wet structures containing five layers with poorly defined interfaces, and thicknesses below the conventional resolution limit, are presented