just ask marcelo cardenas. >> this is one of the spots that we use for -- to hide the guns, the tattoo now making one. >> cardenas, who is sharing his secrets because he is soon leaving for state prison and says he is not worried about punishment, has a clever technique for hiding his homemade tattoo guns. the process begins with another piece of contraband. a box hinge. >> this is my tool that we use to break the wall in by literally scratching it. >> in this case, since the hole is still small, cardenas demonstrates how he uses a bar of soap to conceal one of the magnets used in his guns, rather than the gun itself. >> the thing we do here is we put the magnet inside the hiding spot. we break the soap so you can fix it like -- you have to take more time but right now i'm just showing you. >> cardenas then gently peels small strips of paint off the wall. >> we peel that off a little bit. >> then he moistens the paint with water. >> because this paint is based on water, so the water make it stick. and then you put some paint on top of the soap like this. but then, you know. so you go l