Its somewhat questionable to paint over a conductor to connect the paint. There's almost invariably a discontinuity at the edges and its quite easy for the paint to crack along that discontinuity. If I was doing it cheap without the proper conductive adhesive copper tape, I'd epoxy a piece of thin solderable foil or shim stock to a flat surface of the interior, carefully fairing all edges with a slight excess of epoxy, then sand the middle of the piece back to bare metal, leaving a thin layer of epoxy over the extreme edges. Solder the wire on, de-flux then spray paint.
You'll also see some products that use a leaf or coil spring contact from the PCB to the case.
If the PCB is mounted on moulded in bosses with radiused edges, simply spraying the bosses then, when dry, use Belville or wave spring washers between the mounting screw head and the PCB so that a tinned or plated annular ground ring round the mounting hole is in direct contact with the paint on the boss.