This article develops a novel non-destructive terahertz time-domain polarimetric imaging technique to measure multilayer coating thicknesses and inspect beneath optically opaque surfaces. With a terahertz time-domain polarimetric scanner with the useful bandwidth limited to about 1.5 THz and a training technique based on time-of-flight measurements obtained by sparse deconvolution of the THz time-domain pulses, advanced knowledge of the refractive index of each multilayer coating or the reflectivity of the carbon-fiber-reinforced polymer (CFRP) substrate is unnecessary. Thickness measurements with a mean square error of about 10 µm with 90 accuracy can be achieved by using a simple physics-based linear regression model.