The scanning Kelvin probe (SKP) is a non-destructive technique for measuring the surface distribution of the Volta potential with a high spatial resolution of a few tens of micrometers. The SKP technique allows in situ studies of the localized corrosion processes under atmospheric weathering conditions, on metal surfaces, or underneath organic coatings. In the present study, the SKP technique was used to follow the kinetics of underpaint corrosion from a defect applied on steel coated with thick marine paint systems (0.4 mm to 0.5 mm) as a function of exposure time in an accelerated corrosion test. Three different paint systems were investigated. In addition, the influence of surface cleanliness in terms of salt concentration on a steel substrate prior to paint application was investigated using the SKP technique. The results showed the high efficiency of the SKP technique for early corrosion evaluation under thick paints on steel substrate.