Wet and dry film thickness can be measured non-invasively on structures, such as surfaces associated with vessels, aircraft and buildings, using calibrated microwave sensors. The film is measured by directing microwave energy toward the film. The microwave energy passes through the film and is reflected by a reflective or semi-reflective substrate surface below the film. Properties of the reflected wave are compared with properties of reflected waves that were passed through calibration samples of known thicknesses to determine the unknown thickness of the film. In some embodiments, one or more sensors are maintained at a fixed altitude above the conductive/semi-conductive substrate for measurement, and in other embodiments, one or more sensors are maintained at a fixed altitude above the film. In one embodiment, sensors are associated with a coating applicator, with a first sensor preceding the applicator and a second sensor following the applicator to measure the thickness of the film applied by the applicator by comparing measurements before and after coating.