How do I take paint off a brick, concrete or wooden wall, I want the original paint to show through...?

We(my town) have a vigilante that covers up works of art with grey paint. I want to do him one up and take his paint off leaving the original, is this even possible?How do I take paint off a brick, concrete or wooden wall, I want the original paint to show through...?
a vigilante or a vandal? anyway, taking paint off brick with a stripping solution, like thinner, would work, but getting paint off paint is a tough one. i would begin by using water to see how soluble the undesireable outer paint layer is. then i'd move my way into thinner and then something stronger, like turpentine. do it in an inconspicuous place to test whatever theory you use.