Modern industries increasingly use automatic spraying of heat-resistant Nickel coating with a thickness of T = 1-3 mm for large-size parts made of cast iron with nodular graphite. The process of coating application is characterized by time-dependent behavior of its relative magnetic permeability, μс , that is a function of relaxation time, which can be as long as 24 hours, and by μс deviation from point to point on the surface. Aspects of eddy-current phase method for measuring the T value are considered. The structure of four- winding eddy current transformer transducers is described and results of calculation and optimization of their parameters are presented. The influence of controlled and interfering parameters is considered. Based on the above results, a two-channel combined transducer is developed providing measurement error of ΔТ ≤ ±(0.03T + 0.02) mm in the shop environment in the process of coating application and in the final product check. Results of tests on reference specimens and of application in production processes are presented.