It's certainly possible, but you'd need to have the paint on pretty thick and not be using much torque in tightening down the I/O jacks. If you want to ground through the enclosure, I'd recommend using toothed lock washers on the inside of the enclosure and crank them down securely so that the teeth bite through the paint. Alternatively, you can always run a wire connecting the sleeves of the two I/O jacks together or make sure both jack's sleeves are connected to a ground eyelet on the PCB.
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