The coating can be on the inside of the tube, where it helps to maintain a constant electrical field near the screen. Tubes intended for television use normally have an aluminised interior which acts as a reflector on the back of the screen, and forms one plate of a capacitor. Aquadag is also painted on part of the outside of many tubes. The glass, tube, internal aluminium coating and the outer Aquadag then for a capacitor used to filter the high voltage EHT supply.
Normally there are sprung contacts that touch the Aquadag. If this contact is missing or loose, arcing is often observed at times.
When removing a CRT from a TV set, it is wise to discharge this built-in capacitance to avoid a shock hazard when handling the tube.
Aquadag is a water based colloidal graphite suspension, and may be painted on to items to avoid build up of static, or provide electrical screening.
Aside from its use in the production of CRTs, Aquadag is used in many types of high voltage lab apparatus where the creation of a highly predictable electrostatic field is required. The surfaces of some metals (most notably aluminum) can develop nonconductive oxide layers which tend to disrupt the electrostatic field produced around the surface of the metal when used as an electrode. Aquadag is not subject to such effects, and provides a completely uniform equipotential surface for electrostatics.