Optical terahertz technology has, despite its exciting properties, such as transparency of visibly opaque materials, 30 years after its technological breakthrough, not found a widespread application with societal relevance. Main causes are its maturity and costs. If, however, the uniqueness of both THz radiation and time-domain spectroscopy is used to close a technological gap in the right market sector, we here show that successful applications are in reach. We have chosen the automotive industry, where the optimization of coatings applied in the paint shop is of longstanding concern for this most expensive unit of the car production line. Here we report on the development of a THz-based sensor system. We study the light-matter interaction of drying polymer coatings and use advanced novel signal processing algorithms to determine the state of matter of drying paints. This very first sensor system for the inspection of wet coatings that accurately predicts the eventual dry thickness without requiring paint-type calibration. Industrial robustness is guaranteed by a bespoke optical beam design. Our sensor demonstrator outperforms state-of-the-art thickness sensors applied on dry coatings by a factor three and is expected to become a game changer for the paint shop, where today's scrap rates worldwide are as high as one car body per minute.