Awhile back I think that I posted about paint that blocks wireless communications but apparently that sort of paint that they've been using won't block the new higher frequencies that are starting to be used:
The ability to block electromagnetic (EM) waves comes about when a material's magnetic field resonates at the same frequency as the wave. Wave absorbers are usually made from iron-rich oxides, but higher-frequency transmissions outstrip the power of iron to absorb electromagnetic waves.
However, the standard oxide coating – which contains barium as well as iron – has a maximum resonance frequency that is outstripped at 48 GHz.
Shin-ichi Ohkoshi's team at the University of Tokyo in Japan has just identified a new aluminium-iron oxide able to block waves with a frequency almost four times higher.
The team used a sensitive magnetometer to confirm that a powder of the new oxide can absorb EM waves of up to 182 GHz at room temperature.
http://www.newscientist.com/article/dn1644...t-wireless.html