Nope, not true at all. In the case of a Faraday Cage, which is what we're talking about here, conductivity doesn't matter at all. Or, more accurately, it doesn't matter enough to make any difference in effectiveness. Truly and honestly, it just doesn't matter. Any reasonably conductive metal will completely stop RF from getting in, which is all that you need.
What you are saying is completely untrue. Additional thickness makes a HUGE difference. At one skin depth worth of thickness, you only get about -9 dB attenuation, and a layer of conductive paint is nowhere near the skin depth for any signals that you might care about. Skin depth is dependent upon the conductivity, the magnetic reaction of the material (if applicable), and the frequency you are trying to block. We'll use aluminum for a quick example.
Aluminum foil ranges from 0.004318mm to 0.14986mm. Quick back-of-the-napkin ballpark numbers suggest that the cutoff frequency at that skin depth is somewhere around 1 MHz... maybe only 800 kHz... like I said, a very crude ballpark estimate. This means that the frequency at which you get a 9dB cut lies somewhere in the middle of the AM radio band. Higher frequencies will be attenuated more, lower frequencies less. An awful lot of interference is caused by rectified radio stations, so high attenuation in the AM band is pretty important.
The heavy copper foil I'm suggesting would have a cutoff somewhere in the vicinity of 50-60 kHz. That should block any possibility of a radio station magically getting rectified inside your gear, leaving only pure 60 Hz hum to cause interference. That same point in the middle of the AM band is at about five skin depths in this material because it is about four times as thick and made of copper which is about 1.25 times as conductive (ballpark again). Five skin depths is the point at which a signal is generally considered to be zero (about a 45 dB cut) and is the recommended level of shielding for RFI in electronics.
Conductive paint is damn near useless compared to copper tape. You're lucky to get 50,000 ohms per square even if you get a thick enough coat to be consistent (and you need several coats to even get that). Compare that with pure copper, which is on the order of ten milliohms per square even in coats that are 2 micrometers thick. So copper is five million times as conductive (or, I might need to take the square root, in which case it's 2236 times as conductive... I'm not 100% positive about that).
The skin depth of the AM radio band in conductive paint would therefore be a layer of conductive paint about 330 meters thick, give or take, assuming 50,000 ohms per square. At a more typical 300 kilohms per square, you'd need a layer of paint that is a whopping 1.23 miles thick to get a 9dB drop in the middle of the AM radio band.
Oh, and it decays with the square root of the frequency, so the 9dB drop point of a layer of conductive paint is about 25 exahertz. (25 million million million hertz). Conductive paint is crap. You might as well paint the inside with Krylon.
For some more corroboration, here's a
blind test of hum rejection in cables where the results in order from best rejection to worst were neatly arranged almost precisely in increasing order of shield resistance per unit length.
If you can't detect a difference between shielding paint and copper, it isn't because there is no difference in effectiveness. It's because you aren't in an environment with enough RFI to matter and/or don't have any dirty contacts, etc. inside the guitar acting like a rectifier to tune radio station signals into audible sound. Put yourself in an RF-noisy environment, though, and you'll easily be able to detect the difference. For an easy ten second test, buy a GSM cell phone.
