I've shied away from testing it cos if it does create a horrible buzz it will be a massive pain to clean it all off again
I bought a can of blackboard paint (cheap) some years ago to test that theory. I can't remember how many coats I put on (I did not use an instrument) but it build up thick and did not have electrical continuety. So it's cheap but useless, at least the one I bought.
The sellers of shielding paint use small bottles because most people only need for one or two guitars. Ok the price is high, I've actually been temted to cook up and sell it half the price just to annoy them
but then I'd have to do that all the time, and who'd want that..
It's fairly easy, cheap and well explained how to make your own. If too much hassle - like many said before me - aluminium foil is just as good and very cheap!