Aquadag is a trade name for a water-based colloidal graphite coating commonly used in cathode ray tubes (CRTs).
It can be applied by brushing, swabbing, spraying, or dipping, after which the surface is dried, leaving a layer of pure graphite.
Due to the graphite, it is electrically conductive and forms part of the high-voltage positive electrode, the second anode, which accelerates the electron beam.
It has spring clips, which press against the walls of the tube, making contact with the aquadag coating so it also carries this high positive voltage.
These secondary electrons are attracted to the high positive voltage of the coating and return through it to the anode power supply.
The sandwich of the two coatings separated by the dielectric glass wall of the tube form a final capacitor to filter out ripple from the anode supply.
Aside from its use in the production of CRTs, Aquadag is used in many types of high-voltage lab apparatus where a conductive coating is needed on an insulating surface.