A new dye-doped photocatalytic (TiO2) coating agent, together with a precoating agent, was developed in order to detect debonded regions or gaps in the coating layers during or soon after the coating process. The dyes present in the coating layers act as visible indicators of the coating gaps due to their colors. Various commercial dyes were examined for their detection performance and the bleaching time after the gap detection. The results indicated the suitability of several selective fluorescent dyes for precoating agents and vividly colored dyes for photocatalytic agents. After the detection of the coating gaps, these dyes were bleached by photocatalytic activity of TiO2 over a period of 3 to 7 days under sunlight. Field tests on alumina plates coated with this new photocatalytic coating were found to be successful.