Aquadag (colloidal graphite) can be used to make a highly conductive coating on objects. However, I found it's possible to dilute the Aquadag with PVA glue to produce a high-resistance coating, with better adhesion. The lowest percentage of Aquadag which still seemed to give a conductive coating was around 30%. This is useful for making high-voltage resistors, since they are usually large values anyway and need to be physically large to withstand the voltage.
I experimented with painting the mixture on to some sanded acrylic sheet - it works very well. It's possible to increase the resistance by scoring grooves into the film to form a serpentine path. Resistances are in the 10s to 100s of MΩ. Obviously, power dissipation is a problem because of the insulating plastic substrate, but you could paint it on to an alumina tube which conducts heat better.