Abstract: A reference electrode is rapidly automatically applied to the interior of tapered vitrified zirconia thimbles for an exhaust gas oxygen sensor. The coating is consistently smooth and predetermined in physical and electrical characteristics. In a specific example, a hollow elastomeric finger is nested within the thimble after dispensing a measured quantity of conductive ink into the thimble bottom. A gas jet applied to the ink from the finger lower end flows the conductive ink throughout a coating cavity below a shoulder in the finger after the ink quantity is substantially all applied to cavity walls, it no longer blocks a cavity vent, and allows the gas to flow freely through the cavity. Gas flow is then discontinued, and the finger withdrawn from the thimble.