I don't think you understand Faraday cages. Properly grounded the cage should be able to handle an EMP no problem. The cage creates a shell which is reinforced with stronger signals. The more powerful the blast the more efficient the cage. Many of these paints and wallpapers require proper grounding to work most efficiently.

so it doesn't allow signals out either I suppose.

radio signals bouncing around in a box... hmm reminds me of something.... OH yeah, a fucking microwave

this shit is a potential fire hazard during a solar flare, make sure to only put it on over a fire retardant surface.