A robotic holiday testing apparatus for detecting and marking holidays in the protective coating atop a metal base plate of a storage tank, including: a. a vehicle, b. a power drive unit selectively coupled to a first set of wheels to move the vehicle in the forward or backward direction, and to a second set of wheels to move the vehicle and the right or left direction, c. a holiday detection apparatus having a wettable sponge electrical contact element for establishing an electric circuit with the base plate through each of the holidays, d. a tank for containing an electrolytic fluid supplied into the wettable sponge, and e. a controller which selectively operates the power drive unit to: move the vehicle in the forward direction for selected distance, and f. to move the vehicle in the left or right direction for selected distance, the vehicle thus being movable along a path while the holiday testing apparatus thereon detects and marks holidays for subsequent repair.